20 men completed one-third of a piece of work in 20 days. How many more men should be employed to finish the rest of the work in 25 more days?

Let,
the total number of men be x.
Work done = 1/3,
Remaining work = 1 - (1/3)
= 2/3
More work, More men [Direct proportion]
More days, Less men [Indirect proportion]
{(Work 1/3 : 2/3) (Days 25: 20)} : : 20 : x
∴ {(1/3) × 25 × x } = {(2/3) × 20 × 20}
x = 800/25
= 32
More men to be employed = (32 - 20)
= 12.