Hello !

Usually, you can find in how many days a child finishes the work, etc...

Here's how I'd do. Since each child takes twice the time than a man, we can consider that 2 children are worth 1 man.

Hence 8 children worth 4 men. And thus 14 men would complete the piece of work in 9 days.

14 men -> 9 days

12 men -> ? days

That's a cross product