SOLUTION: Twelve men can complete a work in 8 days. Three days after they started the work, 3 more men joined them. In how many days will all of them together complete the remaining work?

Here's how to work it using "man days", which is "men times days".

So a job requires 12∙8 or 96 man days.

That was 12∙3 or 36 of the required number of 96 man days.

So 96-36 or 60 more man days were needed.

So that made 15 men to complete the remaining 60 man days.

So 60 man days divided by 15 men gives 4 days.

Edwin