pumps working 8 hours a day, can empty a tank in 2 days. How many hours a day must 4 pumps work to empty the tank in 1 day?
Answer: D
Let the required no of working hours per day be x.
More pumps , Less working hours per day (Indirect Proportion)
Less days, More working hours per day (Indirect Proportion)
Pumps4 : 3Days1 : 2⋮⋮ 8:x
=> (4 * 1 * x) = (3 * 2 * 8)
=> x=12