Question

Ben, Tim, and Sam have to do a certain job. Ben can finish the job in 5 hours. Tim can finish the job in 6 hours. Sam can finish the job in x hours. The three of them can finish the job in 2 hours if they work together. Find x.

Answer 1

x is 7.5 hours

Step-by-step explanation:

Ben, Tim, and Sam have to do a certain job

• Ben can finish the job in 5 hours
• Tim can finish the job in 6 hours
• Sam can finish the job in x hours
• The three of them can finish the job in 2 hours if they work together

We need to find x

∵ Ben can finish the job in 5 hours

∴ The rate of Ben = $\frac{1}{5}$

∵ Tim can finish the job in 6 hours

∴ The rate of Tim = $\frac{1}{6}$

∵ Sam can finish the job in x hours

∴ The rate of Sam = $\frac{1}{x}$

They will working together to finish the job in two hours, that means the sum of their rate is equal to 1/2

∵ They can finish the job in 2 hours if they work together

∴ $\frac{1}{5}+\frac{1}{6}+\frac{1}{x}=\frac{1}{2}$

To add the fractions find the LCM for all denominators to make all fractions with same denominators

∵ The LCM of 5, 6 ans x is (5 × 6 × x) = 30 x,

- Divide it by each denominator to find the new numerators

∵ 30 x ÷ 5 = 6 x , 30 x ÷ 6 = 5 x , 30 x ÷ x = 30

- Change all the denominators to 30 x and then write the new

   numerators of each fraction

∴ $\frac{6x}{30x}+\frac{5x}{30x}+\frac{30}{30x}=\frac{1}{2}$

Add the numerators and put the sum over 30 x

∵ $\frac{11x+30}{30x}=\frac{1}{2}$

- By using cross multiplication

∴ 2(11 x + 30) = 1(30 x)

- Simplify the both sides

∴ 22 x + 60 = 30 x

- Subtract 22 x from both sides

∴ 60 = 8 x

- Divide both sides by 8

∴ 7.5 = x

∴ The value of x is 7.5 hours

x is 7.5 hours

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