Two pipes A and B can fill a tank in 30 and 60 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank?  Choices:
A) 10
B) 20
C) 30
D) 24
E) 25
Part filled by A in 1 min. = 1/30   Part filled by B in 1 min. = 1/60   part filled by (A + B) in 1 min. = 1/30 + 1/60 = 1/20.   Both the pipes can fill the tank in 20 minutes.   Answer: 20
<result>B</result>
Two pipes A and B can fill a tank in 30 and 60 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank?