The concept used for this item is equating the man-hours for both scenario. Man-hour is calculated by multiplying the number of men working by the number of hours they need to finish the work. That is,
(15 men)(7 hours) = (35 men)(x hours)
The value of x from the equation is equal to 3. Therefore, it will only take 3 hours for 35 people to do the same job.
