Mathematics
posted by Celina .
Twelve man take 6 hours to finish a piece of work. After the 12 men have worked for 1 hour, the contractor decides to call in 8 men so that the work can be completed earlier. How many more hours would 20 men take to complete the remaining work?

job requires 12(6) = 72 manhours
They work for 1 hour, so 12 manhours are used up, leaving 60 manhours for the job to be done
Second shift has 20 men,
so time needed to complete the job
= 60 manhours/20 men
= 3 hours
(the above applies only in theory, it does not apply to city workers or the like. Doubling the number of men on a job in practise does not cut the required time in half. , (this is supposed to be a joke) ) 
Thanks!