A can do a work in 15 days and B in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is ?
| A's 1 day's work = | 1 | ; |
| 15 |
| B's 1 day's work = | 1 | ; |
| 20 |
| (A + B)'s 1 day's work = |  | 1 | + | 1 |  | = | 7 | . |
| 15 | 20 | 60 |
| (A + B)'s 4 day's work = | | 7 | x 4 | | = | 7 | . |
| 60 | 15 |
| Therefore, Remaining work = | | 1 - | 7 | | = | 8 | . |
| 15 | 15 |
