Question

A water pump takes 70 minutes to fill a tank. A larger pump can fill the same tank in 30 minutes.

How long will it take to fill the tank if both pumps are used?

A. 15 minutes
B. 21 minutes
C. 40 minutes
D. 50 minutes

Answer 1

40 minutes

Explanation:

70 minutes - 30 minutes

Answer 2

21 minutes Option B

Explanation:

1 min

Small --> 1/70

Big --> 1/30

Together --> 1/70 + 1/30 = 3/210 + 7/210 = 10/210 = 1/21

1 divided by 1/21 = 21

Thus, it's 21 minutes (B)