Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank?	Pick.
A) 15 minutes.	B) 17 minutes.	C) 12 minutes.	D) 10 minutes.	E) 11 minutes.
Part filled by A in 1 min =1/20   Part filled by B in 1 min =1/30	.   Part filled by (A + B) in 1 min =1/20+1/30=1/12	.   Both pipes can fill the tank in 12 minutes.   Answer is 10 minutes.
<result>D</result>
Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both the pipes are used together, then how long will it take to fill the tank?