Question 4181
You would use the formula for work which is

1/rate + 1/rate = 1/time

1/4 + 1/6 = 1/t

Add the fraction together.  First you must find a common denominator which is 12.  12 goes into 4 - 3 times and 12 goes into 6 2 times.

3/12 + 2/12 = 5/12= 1/t

Cross Multiply 5/12=1/7 (proportion)

5t=12

The answer will be 2 2/5 hours