Correct option is E
A and B together fill the whole tank = 12 minutes
Let the capacity of tank be 60 units (LCM of 20 & 12)
then, pipe A’s efficiency =60/30=2 units/min
together pipe (A+ B)’s efficiency =60/12 = 5 units/min
so, pipe B’s efficiency = 3 units/min
Pipe B can fill 45 units i.e. 3/4 th part of tank in 15 minutes.