A tank can be filled by a tap in 20 minutes and by another tap in 6O minutes. Both the taps are kept open for 10 minutes and then the first tap is shut off. After this, the tank will be completely filled in:

A. 10 min   
B. 15 min
C. 12 min   
D. 20 min
Answer:     D

Explanation:     

Part filled in 10 min = 10[(1/20) + (1/60)] = 10 * (4/60) = 2/3
Remaining part = (1 - (2/3)) = 1/3
Part filled by second tap in 1 min = 1/60
(1/60) : (1/3) ? 1 : x
Hence, the remaining part will be filled in 20 min.