A pipe can fill a tank in 15 minutes and another one in 10 minutes. A third pipe can empty the tank in 5 minutes. The first two pipes are kept open for 4 minutes in the beginning and then the third pipe is also opened. In what time will the tank be empited ?
Choose the correct choice from the following options:
A) 35 min	B) 15 min	C) 20 min	D) Cannot be emptied	E) None of these
Proportion of the volume of the tank filled by both the   pipes in 4 min = 4(1⁄15 + 1⁄10) = 2⁄3rd of the tank   Volume of the tank filled by all the pipes working together = 1⁄15 + 1⁄10 - 1⁄5 = -1⁄30   i.e. 1⁄30 tank is emptied in 1 min.   ∴ 2⁄3 rd of the tank can be emptied in 2×30/3=20min   Answer 20 min
<result>C</result>
A pipe can fill a tank in 15 minutes and another one in 10 minutes. A third pipe can empty the tank in 5 minutes. The first two pipes are kept open for 4 minutes in the beginning and then the third pipe is also opened. In what time will the tank be empited ?