Fifteen men take 10 days to complete a job working 12 hours a day.How many hours a day should 10 men work to complete the job in 20 days ?

Explanation

Since 15 men take 10 days working 12 hours per day, the total work done measured in terms of man-hours is `15xx1012`.
When 10 men are required to complete the same job in 20 days working h hours a day, work done `=10xx20xxh`. But `10xx20xxh`=15xx10xx12`. Hence the number of hours for which they should work per day is `(15xx10xx12)/(10xx20)=9`.
`:.` 10 men can complete the work in 20 days working for 9 hours per day.
Hence, in general we can say that:
If `M_(1)` men can do `W_(1)` units of work in `D_(1)` days working `H_(1)` hours per day and `M_(2)` men can do
`W_(2)` untis of work in `D_(2)` days working `H_(2)` hours per day, then `(M_(1)D_(1)H_(1))/(W_(1))=(M_(2)D_(2)H_(2))/(W_(2))`