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