Question 699386
One pipe can fill a tank in 2 hours. A second pipe can fill it in 1.75 hours.
 How long will it take to fill the tank with both pipes open?
:
Let t = time required when working together
Let the completed job = 1; (a full tank)
:
Each pipe will do a fraction of the job, the two fractions add up to 1
:
t/2 + t/1.75 = 1
multiply both sides by 14, resulting 
7t + 8t = 14
15t = 14
t = 14/15 hrs
or
t = 14/15 *60 = 56 min working together