Aman 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?

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  of the job.
The finished job = 
The finished job = 
The finished job = 

Determine part of the remaining job
remaining job = 1 - the finished job
remaining job = 1 - 
remaining job = 
The remaining job for the son is 3/4 of the job

Determine time needed to do 3/4 of the job
time needed = 
time needed = 
time needed = 
time needed =  hours
time needed = 2 hours + 15 minutes

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