21.  A cistern has two taps which fill it in 12 minutes and 15 minutes respectively. There is also a waste pipe in the cistern. when all the three are opened, The empty cistern is full in 20 minutes. How long will the waste pipe take to empty the full cistern?

15 minutes
20 minutes
10 minutes
25 minutes

```Answer

Answer: Option  C Explanation:Work done by the waste pipe in 1 minute
= 1/20-(1/12+1/15) = -1/10  (- negative sign means emptying)
waste pipe will empty the full cistern in 10 minutes ```

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22.  An electric pump can fill a tank in 3 hours.Because of a leak in the tank, it took 3 1/2 hours to fill the tank. If the tank is full, how much time will the leak take to empty it?

20 hrs
21 hrs
10 hours
30 hrs

```Answer

Answer: Option  B Explanation:Work done by the leak in 1 hour = (1/3-1/(7/2)) = (1/3 - 2/7) = 1/21
Therefore the leak will empty the tank in 21 hours. ```

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23.  Two pipes A and B can fill a tank in 36 hours and 45 hours respectively.If both the pipes are opened simultaneously, how much time will be taken to fill the tank?

20 hrs
10 hrs
16 hrs
12 hrs

```Answer

Answer: Option  A Explanation:Part filled by A in 1 hour = 1/36; part filled by B in 1 hour =1/45.
part filled by (A+B) in 1 hour = (1/36+1/45) = 9/180 = 1/20.
Hence, both the pipes together will fill the tank in 20 hours. ```

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24.  3 pipes A, B & C can fill a tank in 6 hours. After working at it together for 2 hours, C is closed and A and B can fill the remaining part in 7 hours. The number of hours taken by C alone to fill the tank is?

10
12
14
16

```Answer

Answer: Option  C Explanation:Part filled in 2 hours = 2/6 = 1/3, Remaining part = (1- 1/3) = 2/3.
Therefore (A+B)'s 7 hours work = 2/3: (A+B)'s 1 hour's work =2/21.
Therefore C's 1 hour's work = {(A+B+C)'s 1 hour's work - (A+B)'s 1 hour's work}
=(1/6-2/21)=1/14
Therefore C alone can fill the tank in 14 hours.
t ```

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25.  2 pipes A & B can fill a cistern in 37 1/2 minutes and 45 minutes respectively.Both pipes are opened.The cistern will be filled in just half an hour.If the pipe B is turned off after?

5 mins
9 mins
10 mins
15 mins

```Answer

Answer: Option  B Explanation:Let B be turned off after x mins. Then
part filled by (A+B) - in x minute. + part filled by A in (30 -x ) minutes =1.
Therefore X ( 2/75+1/45) + (30-x) 2/75 =1
11x/225 + (60-2x) / 75 =1 = 11x + 180 - 6x = 225 = x=9 ```

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