Two pipes A and B can fill a tank in 3 and 6 min respectively.Starting with A both the pipes are opened alternatively each for 1 min.In what time the tank will be filled?

fraction of tank filled by A per minute

= 1/3
fraction of tank filled by B per minute
= 1/6
Fraction of tank filled in first two minutes
= 1/3+1/6 = 1/2
Can you take it from here?