15 men can complete a work in 18 days by working 8 hours per day. How many men will be required to complete the same work in 6 days by working 12 hours per day?

30 men are required.

The work is fixed, so men x days x hours stays constant.

15 x 18 x 8 = 2160 man-hours.

Set M x 6 x 12 = 2160.

72 M = 2160, so M = 30.

This uses the work equivalence M1 D1 H1 = M2 D2 H2.