Two fill taps A and B can separately fill a cistern in 45 and 40 minutes respectively. They started to fill a cistern together but tap A is turned off after few minutes and tap B fills the rest part of cistern in 23 minutes. After how many minutes,was tap A turned-off?	Choose the correct choice from the following options
A) 9 min
B) 10 min
C) 12 min
D) 7 min
E) none of these

Rationale: let a was turned off after x min . then , cistern filled by a in x min + cistern filled by b in ( x + 23 ) min = 1 ⇒ x / 45 + x + 23 / 40 = 1 ⇒ 17 x + 207 = 360 ⇒ x = 9 min . answer a
Answer: A