3 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 ?
Choose one:
A) 9
B) 10
C) 11
D) 12
E) 13
Solution   Let the required number 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)   ∴ 4 × 1 × x = 3 × 2 × 8 ⇔ x = 3x2x8/4 = x= 12, Answer 12
<result>D</result>
3 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 ?