Using a Problem-Solving Strategy to Solve Number Problems

Learning Outcomes

Apply the general problem-solving strategy to number problems
Identify how many numbers you are solving for given a number problem
Solve consecutive integer problems

Now we will translate and solve number problems. In number problems, you are given some clues about one or more numbers, and you use these clues to build an equation. Number problems don’t usually arise on an everyday basis, but they provide a good introduction to practicing the Problem-Solving Strategy. Remember to look for clue words such as difference, of, and and.

Example

The difference of a number and six is [latex]13[/latex]. Find the number.

Solution:

Step 1. Read the problem. Do you understand all the words?
Step 2. Identify what you are looking for.	the number
Step 3. Name. Choose a variable to represent the number.	Let [latex]n=\text{the number}[/latex]
Step 4. Translate. Restate as one sentence. Translate into an equation.	[latex]n-6\enspace\Rightarrow[/latex] The difference of a number and 6 [latex]=\enspace\Rightarrow[/latex] is [latex]13\enspace\Rightarrow[/latex] thirteen
Step 5. Solve the equation. Add 6 to both sides. Simplify.	[latex]n-6=13[/latex] [latex]n-6\color{red}{+6}=13\color{red}{+6}[/latex] [latex]n=19[/latex]
Step 6. Check: The difference of [latex]19[/latex] and [latex]6[/latex] is [latex]13[/latex]. It checks.
Step 7. Answer the question.	The number is [latex]19[/latex].

try it

example

The sum of twice a number and seven is [latex]15[/latex]. Find the number.

Show Solution

try it

Watch the following video to see another example of how to solve a number problem.

Solving for Two or More Numbers

Some number word problems ask you to find two or more numbers. It may be tempting to name them all with different variables, but so far we have only solved equations with one variable. We will define the numbers in terms of the same variable. Be sure to read the problem carefully to discover how all the numbers relate to each other.

example

One number is five more than another. The sum of the numbers is twenty-one. Find the numbers.

Show Solution

try it

Watch the following video to see another example of how to find two numbers given the relationship between the two.

example

The sum of two numbers is negative fourteen. One number is four less than the other. Find the numbers.

Show Solution

try it

example

One number is ten more than twice another. Their sum is one. Find the numbers.

Show Solution

try it

Solving for Consecutive Integers

Consecutive integers are integers that immediately follow each other. Some examples of consecutive integers are:

[latex]\begin{array}{c}\phantom{\rule{0.2}{0ex}}\\ \phantom{\rule{0.2}{0ex}}\\ \phantom{\rule{0.2}{0ex}}\\ \phantom{\rule{0.2}{0ex}}\\ \hfill \text{...}1,2,3,4\text{,...}\hfill \end{array}[/latex]
[latex]\text{...}-10,-9,-8,-7\text{,...}[/latex]
[latex]\text{...}150,151,152,153\text{,...}[/latex]

Notice that each number is one more than the number preceding it. So if we define the first integer as [latex]n[/latex], the next consecutive integer is [latex]n+1[/latex]. The one after that is one more than [latex]n+1[/latex], so it is [latex]n+1+1[/latex], or [latex]n+2[/latex].

[latex]\begin{array}{cccc}n\hfill & & & \text{1st integer}\hfill \\ n+1\hfill & & & \text{2nd consecutive integer}\hfill \\ n+2\hfill & & & \text{3rd consecutive integer}\hfill \end{array}[/latex]

example

The sum of two consecutive integers is [latex]47[/latex]. Find the numbers.

Solution:

Step 1. Read the problem.
Step 2. Identify what you are looking for.		two consecutive integers
Step 3. Name.		Let [latex]n=\text{1st integer}[/latex] [latex]n+1=\text{next consecutive integer}[/latex]
Step 4. Translate. Restate as one sentence. Translate into an equation.		[latex]n+n+1\enspace\Rightarrow[/latex] The sum of the integers [latex]=\enspace\Rightarrow[/latex] is [latex]47\enspace\Rightarrow[/latex] 47
Step 5. Solve the equation.		[latex]n+n+1=47[/latex]
Combine like terms.		[latex]2n+1=47[/latex]
Subtract 1 from each side.		[latex]2n=46[/latex]
Divide each side by 2.		[latex]n=23[/latex] 1st integer
Substitute to get the second number.		[latex]n+1[/latex] 2nd integer
		[latex]\color{red}{23}+1[/latex]
		[latex]24[/latex]
Step 6. Check:	[latex]23+24\stackrel{\text{?}}{=}47[/latex] [latex]47=47\quad\checkmark[/latex]
Step 7. Answer the question.		The two consecutive integers are [latex]23[/latex] and [latex]24[/latex].

try it

example

Find three consecutive integers whose sum is [latex]42[/latex].

Show Solution

try it

Watch this video for another example of how to find three consecutive integers given their sum.

Step 1. Read the problem.
Step 2. Identify what you are looking for.	the number
Step 3. Name. Choose a variable to represent the number.	Let [latex]n=\text{the number}[/latex]
Step 4. Translate. Restate the problem as one sentence. Translate into an equation.	[latex]2n\enspace\Rightarrow[/latex] The sum of twice a number [latex]+\enspace\Rightarrow[/latex] and [latex]7\enspace\Rightarrow[/latex] seven [latex]=\enspace\Rightarrow[/latex] is [latex]15\enspace\Rightarrow[/latex] fifteen
Step 5. Solve the equation.	[latex]2n+7=15[/latex]
Subtract 7 from each side and simplify.	[latex]2n=8[/latex]
Divide each side by 2 and simplify.	[latex]n=4[/latex]
Step 6. Check: is the sum of twice [latex]4[/latex] and [latex]7[/latex] equal to [latex]15[/latex]? [latex]2\cdot{4}+7=15[/latex] [latex]8+7=15[/latex] [latex]15=15\quad\checkmark[/latex]
Step 7. Answer the question.	The number is [latex]4[/latex].

Step 1. Read the problem.
Step 2. Identify what you are looking for.		You are looking for two numbers.
Step 3. Name. Choose a variable to represent the first number. What do you know about the second number? Translate.		Let [latex]n=\text{1st number}[/latex] One number is five more than another. [latex]x+5={2}^{\text{nd}}\text{number}[/latex]
Step 4. Translate. Restate the problem as one sentence with all the important information. Translate into an equation. Substitute the variable expressions.		The sum of the numbers is [latex]21[/latex]. The sum of the 1st number and the 2nd number is [latex]21[/latex]. [latex]n\enspace\Rightarrow[/latex] First number [latex]+\enspace\Rightarrow[/latex] + [latex]n+5\enspace\Rightarrow[/latex] Second number [latex]=\enspace\Rightarrow[/latex] = [latex]21\enspace\Rightarrow[/latex] 21
Step 5. Solve the equation.		[latex]n+n+5=21[/latex]
Combine like terms.		[latex]2n+5=21[/latex]
Subtract five from both sides and simplify.		[latex]2n=16[/latex]
Divide by two and simplify.		[latex]n=8[/latex] 1st number
Find the second number too.		[latex]n+5[/latex] 2nd number
Substitute [latex]n = 8[/latex]		[latex]\color{red}{8}+5[/latex]
		[latex]13[/latex]
Step 6. Check:
Do these numbers check in the problem? Is one number 5 more than the other? Is thirteen, 5 more than 8? Yes. Is the sum of the two numbers 21?	[latex]13\stackrel{\text{?}}{=}8+5[/latex] [latex]13=13\quad\checkmark[/latex] [latex]8+13\stackrel{\text{?}}{=}21[/latex] [latex]21=21\quad\checkmark[/latex]
Step 7. Answer the question.		The numbers are [latex]8[/latex] and [latex]13[/latex].

Step 1. Read the problem.
Step 2. Identify what you are looking for.		two numbers
Step 3. Name. Choose a variable. What do you know about the second number? Translate.		Let [latex]n=\text{1st number}[/latex] One number is [latex]4[/latex] less than the other. [latex]n-4={2}^{\text{nd}}\text{number}[/latex]
Step 4. Translate. Write as one sentence. Translate into an equation. Substitute the variable expressions.		The sum of two numbers is negative fourteen. [latex]n\enspace\Rightarrow[/latex] First number [latex]+\enspace\Rightarrow[/latex] + [latex]n-4\enspace\Rightarrow[/latex] Second number [latex]=\enspace\Rightarrow[/latex] = [latex]-14\enspace\Rightarrow[/latex] -14
Step 5. Solve the equation.		[latex]n+n-4=-14[/latex]
Combine like terms.		[latex]2n-4=-14[/latex]
Add 4 to each side and simplify.		[latex]2n=-10[/latex]
Divide by 2.		[latex]n=-5[/latex] 1st number
Substitute [latex]n=-5[/latex] to find the 2^nd number.		[latex]n-4[/latex] 2nd number
		[latex]\color{red}{-5}-4[/latex]
		[latex]-9[/latex]
Step 6. Check:
Is −9 four less than −5? Is their sum −14?	[latex]-5-4\stackrel{\text{?}}{=}-9[/latex] [latex]-9=-9\quad\checkmark[/latex] [latex]-5+(-9)\stackrel{\text{?}}{=}-14[/latex] [latex]-14=-14\quad\checkmark[/latex]
Step 7. Answer the question.		The numbers are [latex]−5[/latex] and [latex]−9[/latex].

Step 1. Read the problem.
Step 2. Identify what you are looking for.		two numbers
Step 3. Name. Choose a variable. One number is ten more than twice another.		Let [latex]x=\text{1st number}[/latex] [latex]2x+10={2}^{\text{nd}}\text{number}[/latex]
Step 4. Translate. Restate as one sentence.		Their sum is one.
Translate into an equation		[latex]x+(2x+10)\enspace\Rightarrow[/latex] The sum of the two numbers [latex]=\enspace\Rightarrow[/latex] is [latex]1\enspace\Rightarrow[/latex] 1
Step 5. Solve the equation.		[latex]x+2x+10=1[/latex]
Combine like terms.		[latex]3x+10=1[/latex]
Subtract 10 from each side.		[latex]3x=-9[/latex]
Divide each side by 3 to get the first number.		[latex]x=-3[/latex]
Substitute to get the second number.		[latex]2x+10[/latex]
		[latex]2(\color{red}{-3})+10[/latex]
		[latex]4[/latex]
Step 6. Check.
Is 4 ten more than twice −3? Is their sum 1?	[latex]2(-3)+10\stackrel{\text{?}}{=}4[/latex] [latex]-6+10=4[/latex] [latex]4=4\quad\checkmark[/latex] [latex]-3+4\stackrel{\text{?}}{=}1[/latex] [latex]1=1\quad\checkmark[/latex]
Step 7. Answer the question.		The numbers are [latex]−3[/latex] and [latex]4[/latex].

Step 1. Read the problem.
Step 2. Identify what you are looking for.		three consecutive integers
Step 3. Name.		Let [latex]n=\text{1st integer}[/latex] [latex]n+1=\text{2nd consecutive integer}[/latex] [latex]n+2=\text{3rd consecutive integer}[/latex]
Step 4. Translate. Restate as one sentence. Translate into an equation.		[latex]n\enspace +\enspace n+1\enspace +\enspace n+2\enspace\Rightarrow[/latex] The sum of the three integers [latex]=\enspace\Rightarrow[/latex] is [latex]42\enspace\Rightarrow[/latex] 42
Step 5. Solve the equation.		[latex]n+n+1+n+2=42[/latex]
Combine like terms.		[latex]3n+3=42[/latex]
Subtract 3 from each side.		[latex]3n=39[/latex]
Divide each side by 3.		[latex]n=13[/latex] 1st integer
Substitute to get the second number.		[latex]n+1[/latex] 2nd integer
		[latex]\color{red}{13}+1[/latex]
		[latex]24[/latex]
Substitute to get the third number.		[latex]n+2[/latex] 3rd integer
		[latex]\color{red}{13}+2[/latex]
		[latex]15[/latex]
Step 6. Check:	[latex]13+14+15\stackrel{\text{?}}{=}42[/latex] [latex]42=42\quad\checkmark[/latex]
Step 7. Answer the question.		The three consecutive integers are [latex]13[/latex], [latex]14[/latex], and [latex]15[/latex].

Module 5: Multi-Step Linear Equations