Use unitary method...

Take total work = 1
3 machines take 36 hours to complete the entire work = 1
1 machine take 36 hours to complete = 1/3 work
1 machine take 1 hour to complete = 1/ 3 * 36
4 machines take 1 hour to complete = 4/3 *36

4 machines complete the work in = 3 * 36 / 4 = 27 hours

4 machines take (36 - 27)= 9 hours less to complete the work.

Please give me KUDO s if you liked my explanation.

If 3 machines working together take 36 hours to complete a job, then 1 machine by itself will take 3 times as long, or 3 x 36 = 108 hours to complete the same job. However, 4 machines working together will take ¼ as long, or ¼ x 108 = 27 hours to complete the job. Therefore, 1 additional machine (added to 3 machines) will save 36 - 27 = 9 hours.

Answer: B
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There are multiple approaches to solve this Question. Let’s try a simple approach.
Since it is given that each machine work at the same constant rate, we can assume that each machine will do 1 order in 1 hour.
No of orders done by 1 machine in 36 hours = 36 orders
Total no of orders done by 3 machines in 36 hours = 36* 3 orders
If we add one more machine, then there is a total of 4 machines
Time taken by 4 machines to do the same no of orders = 36*3/4 = 9* 3 = 27 hours.

So four machines working together can fulfill the orders in 9 hours less time.

Option B is the answer.

Thanks,
Clifin J Francis
GMAT SME
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