Question 276455
(40+10)(x-25)=40x
let x represent the initial amount of men on the job.
The key to this problem is simply understanding 
Work Mandays --yes Mandays problems!
50x-1250=40x
10x=1250
x=125

number of men TIMES number of days EQUALS MANDAYS
                    x                        40                                 = (x-25)(40+10)=Same Mandays, or same amount of time      

to finish the work.