Question

by working 8 hour a day 13 farmers can plough a field in day 15 day . in how many day will 26 farmer plough the same field if they work 10 hours a day
Steve
The amount of work is

8hr/day * 13men * 15day = 1560 man-hr

So, you want x so that

10*26*x = 1560
x = 6 days