A large tank can filled by A and B in 60 minutes and 40 minutes respectively. How many minutes will it take to fill the tanker from empty state if B is used for half the time and A and B fill it together for the other half?	Answers:  A) 10 min  B) 15 min  C) 30 min  D) 25 min  E) 42 min

Rationale: part filled by a + b in 1 minute = 1 / 60 + 1 / 40 = 1 / 24 suppose the tank is filled in x minutes then , x / 2 ( 1 / 24 + 1 / 40 ) = 1 x / 2 * 1 / 15 = 1 x = 30 min answer is c
Answer: C