Two pipes a and b can fill a tank in 12 hours and 15 hours respectively.if both pipe open together in tank,then how much time it take to fill the tank?	Select the correct option:
A) 15	B) 18	C) 20 / 3	D) 19 / 4	E) 25 / 4

Rationale: in 1 hour pipe a fill = 1 / 12 in 1 hour pipe b fill = 1 / 15 pipe ( a + b ) in 1 hour = [ 1 / 12 + 1 / 15 ] = 3 / 20 total time taken to fill the tank = 20 / 3 hour answer c
Answer: C