GMATCoachBen
During each shift at a certain factory, nails are manufactured at a constant rate of 60 nails per 5 seconds and packaged at a constant rate of 100 nails per 9 seconds. If there are 1,000 nails that are
not packaged at the start of a certain shift and if each shift lasts more than 2 hours, how many nails are there that are
not packaged 2 hours after the start of the shift?
A) 2,000
B) 3,200
C) 4,500
D) 6,000
E) 7,400
Always make sure we understand exactly what it's asking for:
"how many nails are there that are not packaged 2 hours after the start of the shift?"Manufacturing rate: 60 nails / 5 seconds = 12 n/s
Packaging rate: 100n / 9s = 11\(\frac{1}{9}\) n/s
We see that they are being manufactured faster than they are being packaged, so the number NOT packaged will increase from the starting value of 1,000. This is also reflected in all of the answer choices.
The rate of increase in the number not packaged will be the difference between these 2 rates:
12 n/s - 11\(\frac{1}{9}\) n/s = \(\frac{8}{9}\) n/s
The time is 2 hours. We need to convert the hours to seconds, and multiply this Rate * Time to get the increase in nails not packaged:
\(\frac{8}{9}\) \(\frac{n}{s}\) * 2 hours * 3600 \(\frac{s}{hour}\) = 6400 increase in nails not packaged
Finally, we must remember to add the starting value of 1,000 nails that are not packaged at the start of the shift. (In general, Starting Value + Change = Final Value.) It's always a great habit to confirm what it's asking for.
1000 + 6400 =
7400