Two pipes A and B can fill a tank in 5 hrs and 20 hrs respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?  Answers:
A) 5 hr
B) 4 hr
C) 6 hr
D) 3 hr
E) 7 hr
Explanation:   Pipe A can fill 1/5 of the tank in 1 hr   Pipe 4 hr can fill 1/20 of the tank in 1 hr   Pipe A and 4 hr together can fill 1/5+1/20=1/4 of the tank in 1 hr   i.e., Pipe A and 4 hr together can fill the tank in 4 hours   Answer: Option 4 hr
<result>B</result>
Two pipes A and B can fill a tank in 5 hrs and 20 hrs respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?