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A and B can do a piece of work in 20 days and 12 days respectively. A started the work alone and then after 4 days B joined him till the completion of the work. How long did the work last ?
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- 10 days
- 20 days
- 15 days
- 6 days
Correct Option: A
| A’s 1 day’s work = | 20 |
| A’s 4 days’ work = | = | |||
| 20 | 5 |
| Remaining work = 1 - | = | |||
| 5 | 5 |
This part is completed by A and B together.
Now, (A + B)’s 1 day’s work
| = | + | |||
| 20 | 12 |
| = | = | = | ||||
| 60 | 60 | 15 |
| Now, | work is done by (A + B) in 1 day. | 15 |
| ∴ | work is done in | 5 |
| = | × | = 6 days | ||
| 2 | 5 |
Hence, the work lasted for 4 + 6 = 10 days.
