Hi,

The best approach to this problem is to think about is to think about the two machines' relative rates. The first machine can complete 1/8 of the task in an hour (if it takes 8 hours to do the job, then the machine can get 1/8 done in one hour), and the second machine can complete ¼ (or 2/8) of the task in the same hour (similar logic as above). We can then combine 2/8+1/8 and figure out that in a single hour, the two machines can get 3/8 of the job done.

So, if the machines together get 3/8 of the job done in an hour, and the second machine accomplishes 2/8 of the job, and the first machine accomplishes 1/8 of the job, then machine b represents 2/3 of the total work done (2/8 divided by 3/8 = 2/3, or 66 2/3%.)


Alternatively, you could consider it from the aspect of getting the jobs done. If both machines work for 8 hours, how many times would the task get done? The answer is 3, the first machine could do it once in 8 hours, the second machine could do it twice in 8 hours (four hours for the first, four more for the second). So if a task the size n could get done 3 times in eight hours, and the second machine does 2 of those 3 times, then the answer is 2/3 (66 2/3%)

Correct answer is D.

dbowden wrote:Hi,

Can someone help me solve this question?

Working alone at its own constant rate, a machine seals n cartons in 8 hours, and working alone at its own constant rate, a second machine seals n cartons in 4 hours. If the two machines, each working at its own constant rate and for the same period of time, together sealed a certain number of cartons, what percent of the cartons were sealed by the machine working at the faster rate?

(A) 25%
(B) 33 1/3%
(C) 50%
(D) 66 2/3%
(E) 75%

Time and rate are RECIPROCALS.
The TIME RATIO for the two machines = slower time : faster time = 8:4.
Thus, the RATE RATIO for the two machines = slower rate : faster rate = 4:8.
Implication: for every 4 cartons sealed by the slower machine, the faster machine seals 8 cartons.
Thus, of every 12 cartons, 8 are sealed by the faster machine:
8/12 = 2/3 = 66 2/3 %.

The correct answer is D.