Got stuck on this GMAT question, can anyone help me out?

Question

Five machines at a certain factory operate at the same constant rate. If four of these machines, operating simultaneously, take 30 hours to fill a certain production order, how many fewer hours does it take all five machines, operating simultaneously, to fill the same production order?
A. 3 
B. 5 
C. 6 
D. 16
E. 24

Aarushi S · Accepted Answer

This is a classic GMAT work-rate question. The key is to understand that total work = machines &times; time.
If 4 machines take 30 hours, the total work is 4 &times; 30 = 120 machine-hours. That&rsquo;s the full job. Now if 5 machines do the same 120 machine-hours of work, divide 120 by 5,  it takes 24 hours.
So, 5 machines finish the job 6 hours faster than 4 machines. That means the correct answer is C.
What often trips people up here is confusing individual machine time with total time. Just remember that &ldquo;machine-hours&rdquo; tells you the actual work done, not just how long one machine runs. Once that&rsquo;s clear, these types of problems become much easier to crack.

kritika K · Answer

This looks harder than it is ,it's just a rate and work problem hiding behind machine numbers.

Four machines &times; 30 hours = 120 machine-hours of work.
If 5 machines do the same 120 machine-hours of work, then: 120 &divide; 5 = 24 hours.
That&rsquo;s 6 hours less than 30. So, answer is C.

Another way to think about it: adding one more identical machine increases the work capacity by 25%. So you&rsquo;d expect the total time to drop by around 20%, which checks out here (30 &rarr; 24 hours).
It helps to jot down &ldquo;work = rate &times; time&rdquo; early on in these problems &mdash; many GMAT quant questions just play around with that core equation