Two taps A and B can fill a cistern in 12 minutes and 18 minutes respectively. They are turned on at the same time. If the tap A is turned off after 4 minutes, how long will tap B take to fill the rest of the cistern ?	Choices:
A) 8 minute
B) 9 min.
C) 10 min.
D) 7 min.
E) None of these
In one min, (A + B) fill the cistern = 1⁄12 + 1⁄18 = 5⁄36th   In 4 min, (A + B) fill the cistern = 5⁄36 × 4 = 5⁄9th   Rest part = 1 - 5⁄9 = 4⁄9th   ∵ 1⁄18th part is filled by B in one min.   ∴ 4⁄9th part is filled by B in 18 × 4⁄9 = 8 min   8 minutenswer 8 minute
<result>A</result>
Two taps A and B can fill a cistern in 12 minutes and 18 minutes respectively. They are turned on at the same time. If the tap A is turned off after 4 minutes, how long will tap B take to fill the rest of the cistern ?