Hello, I'm having trouble working with this problem:

A capentry work can be done in 3 days by 12 persons. If 9 persons work on the same task, how long would it take them to finish it?

The correct answer is 4, but I don't understand how to get it.

I thought I had to do: 9 x 3 / 12, but it's not correct.

How can I solve this?

Thanks