math
posted by thomas .
A father can do a job as fast as two sons working together. If one sone does the job alone in three hours and the other does it alone in six hours, how many hours does it take the father to do the job alone?
a. 1
b. 2
c. 3
d. 4
e. 4 1/2
please answer and explain

1/x = 1/3 + 1/6
x = 2
So, (B)
Extra credit: how long does it take if all three work together?