A contractor agreeing to finish a work in 150 days, employed 75 men each working 8 hours daily. After 90 days, only 2/7 of the work was completed. Increasing the number of men by ________ each working now for 10 hours daily, the work can be completed in time.

One day's work = 2 / (7 * 90)
One hour's work = 2 / (7 * 90 * 8)
One man's work = 2 / (7 * 90 * 8 * 75)

The remaining work (5/7) has to be completed within 60 days, because the total number of days allotted for the project is 150 days.So we get the

equation(2 * 10 * x * 60) / (7 * 90 * 8 * 75) = 5/7

where x is the number of men working after the 90th day.

We get x = 225

Since we have 75 men already, it is enough to add only 150 men..