Discussion
Two pipes A and B can fill a tank in 15 minutes and 20 minutes respectively. Both the pipes are opened together but after 4 minutes, pipe A is turned off. What is the total time required to fill the tank?
Answer: D
| Part filled in 4 minutes = 4 |  |
1 | + | 1 |  |
= | 7 | . |
| 15 | 20 | 15 |
| Remaining part = | |
1 - | 7 | |
= | 8 | . |
| 15 | 15 |
| Part filled by B in 1 minute = | 1 |
| 20 |
 |
1 | : | 8 | :: 1 : x |
| 20 | 15 |
| x = | |
8 | x 1 x 20 | |
= 10 | 2 | min = 10 min. 40 sec. |
| 15 | 3 |
The tank will be full in (4 min. + 10 min. + 40 sec.) = 14 min. 40 sec.
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