Question

A man can mow his lawn in 4 hours. His son can do the job in 3 hours. If the son takes over an hour after his father, how long will it take him to finish mowing the lawn?

Answer 1

Determine part of the job that has been done by the father.
The father did for an hour before the son took over, and he could complete the job within 4 hours. That means he already did $\frac{1}{4}$ of the job.
The finished job = $\dfrac{\text{time of work}}{\text{time needed to do whole job}}$
The finished job = $\dfrac{\text{1 hour}}{\text{4 hours}}$
The finished job = $\dfrac{1}{4}$

Determine part of the remaining job
remaining job = 1 - the finished job
remaining job = 1 - $\frac{1}{4}$
remaining job = $\frac{3}{4}$
The remaining job for the son is 3/4 of the job

Determine time needed to do 3/4 of the job
time needed = $\frac{3}{4} \times \text{time needed to do whole job}$
time needed = $\frac{3}{4} \times 3$
time needed = $\frac{9}{4}$
time needed = $2 \frac{1}{4}$ hours
time needed = 2 hours + 15 minutes

To finish mowing the lawn, the son needs 2 hours and 15 minutes