9514 1404 393
Answer:
42 6/7 minutes
Step-by-step explanation:
In each minute, the first pump adds 1/50 of a tankful, the second adds 1/60, and the third subtracts 1/75. Then at the end of each minute, the amount added to the tank is ...
1/50 +1/60 -1/75 = (6 +5 -4)/300 = 7/300 . . . tankful
The time required to fill the tank is the reciprocal of this rate:
1/(7/300 tankful/minute) = 300/7 minutes/tankful
300/7 minutes = 42 6/7 minutes
