It takes a man 5 hours to wash and wax the family car. if his teenage son him, it only takes 1 hour. how long would it take the son, working alone, to wash and wax the car?

In one hour the father can wash 1/5 of the car while the remaining 4/5 is done by the son.

to do 1/5 wash & wax, the son requires 1/4 hours (i.e 15 mins)
Therefore, the son requires one hour and 15 minutes to wash and wax the car by himself.

Hope that helps.

The answer would be (c)