Let's solve each of the word problems step by step using
linear equations.
---
Problem 1:
> There are two numbers whose sum is 72. One number is twice the other. What are the numbers?
Step 1: Let the smaller number be $ x $.
Then the other number (twice the first) is $ 2x $.
Step 2: Their sum is 72:
$$
x + 2x = 72
$$
$$
3x = 72
$$
$$
x = 24
$$
So, the numbers are:
- $ x = 24 $
- $ 2x = 48 $
✔ Answer: The numbers are
24 and 48.
---
Problem 2:
> Clarke is 8 years older than John. In 5 years Clarke will be twice as old as John. How old are they now?
Step 1: Let John’s current age be $ x $.
Then Clarke’s current age is $ x + 8 $.
Step 2: In 5 years:
- John’s age: $ x + 5 $
- Clarke’s age: $ x + 8 + 5 = x + 13 $
According to the problem:
$$
x + 13 = 2(x + 5)
$$
Solve:
$$
x + 13 = 2x + 10
$$
$$
13 - 10 = 2x - x
$$
$$
3 = x
$$
So:
- John is $ 3 $ years old
- Clarke is $ 3 + 8 = 11 $ years old
✔ Answer: John is
3, Clarke is
11.
---
Problem 3:
> Find three consecutive odd integers such that the sum of the first, two times the second, and three times the third is 70.
Step 1: Let the three consecutive odd integers be:
- First: $ x $
- Second: $ x + 2 $
- Third: $ x + 4 $
Step 2: Set up the equation:
$$
x + 2(x + 2) + 3(x + 4) = 70
$$
Simplify:
$$
x + 2x + 4 + 3x + 12 = 70
$$
$$
6x + 16 = 70
$$
$$
6x = 54
$$
$$
x = 9
$$
So the numbers are:
- $ x = 9 $
- $ x + 2 = 11 $
- $ x + 4 = 13 $
Check:
$$
9 + 2(11) + 3(13) = 9 + 22 + 39 = 70
✔
$$
✔ Answer: The integers are
9, 11, 13.
---
Problem 4:
> Tim sold half of his story books and then bought 6 more. He now has 12. How many did he begin with?
Step 1: Let the original number of books be $ x $.
He sold half: $ \frac{1}{2}x $ left.
Then he bought 6 more:
$$
\frac{1}{2}x + 6 = 12
$$
Solve:
$$
\frac{1}{2}x = 6
$$
$$
x = 12
$$
✔ Answer: Tim began with
12 books.
---
Problem 5:
> Sara can do a job in 12 hours. If Tom helps, it takes them 8 hours. Without help, how long would it take for Tom to do the same work?
This is a
work rate problem.
Step 1: Let’s define rates:
- Sara’s rate: $ \frac{1}{12} $ jobs per hour
- Tom’s rate: $ \frac{1}{t} $ jobs per hour (we want to find $ t $)
- Combined rate: $ \frac{1}{8} $ jobs per hour
Step 2: Add their rates:
$$
\frac{1}{12} + \frac{1}{t} = \frac{1}{8}
$$
Solve for $ t $:
Subtract $ \frac{1}{12} $ from both sides:
$$
\frac{1}{t} = \frac{1}{8} - \frac{1}{12}
$$
Find common denominator (24):
$$
\frac{1}{8} = \frac{3}{24},\quad \frac{1}{12} = \frac{2}{24}
$$
$$
\frac{1}{t} = \frac{3}{24} - \frac{2}{24} = \frac{1}{24}
$$
So:
$$
t = 24
$$
✔ Answer: It would take Tom
24 hours to do the job alone.
---
✔ Final Answers Summary:
1.
24 and 48
2.
John: 3 years, Clarke: 11 years
3.
9, 11, 13
4.
12 books
5.
24 hours
Let me know if you'd like these explained in a different way or visualized!
Parent Tip: Review the logic above to help your child master the concept of 3 variable system of equations word problems worksheet.