Question

in order to complete a work in 28 days 60 men are required how many men will be required it the same work is to be completed in 40 days

Answer 1

Let there are 28×60 parts of that work = 1680 parts
in 40 days, men needed = (total work)/days = 1680/40 = 42 men
Method 2
work is always the same, so
men1×days1 = men2×days2
⇒28×60= men2×40
⇒men2 = 42 

Answer 2

It is inverse proportion

Let the number of workers reqd. to complete the work in 40 days be 'x'.

28×60 = 40 × x
(Since its inversely proportional)

1680/40= x
x = 42 workers