It takes 4 men and 3 hours to pave a playground.

More men will finish the playground faster - less time (inverse proportion)

If we have 3 times more men that is 4*3=12 men the playground will be finished 3 times faster then before, that is, in 3 / 3 = 1 hour.

Or we can write this down like this:

4 : 12 = x : 3 (inverse proportion)

12 * x = 4 * 3

12* x = 12

x = 1 hour

12 men will complete the task in 1 hour time.