rate and average questions

[lalabee]

Question: G can copy 50 pages in 8 hrs, and S copy 50 pages in 6 hours. to copy 100 page together, they will use how many hours?

===>> (50/8+50/6)*time=100

Question: G drives 50 miles in 8 hrs, and S drives 50 miles in 6 hrs. To drive 100 miles together, they will use how many hours?

It seems like we can combine rates, but we cant do the same thing for average speed? Can anyone explain to me why?


Thanks a lot.

[simplyjat]

To drive 100 miles together

What do you mean by driving together?. In English it means each one drives 100 miles....

[Stuart@KaplanGMAT]

Question: G can copy 50 pages in 8 hrs, and S copy 50 pages in 6 hours. to copy 100 page together, they will use how many hours?

===>> (50/8+50/6)*time=100

Question: G drives 50 miles in 8 hrs, and S drives 50 miles in 6 hrs. To drive 100 miles together, they will use how many hours?

It seems like we can combine rates, but we cant do the same thing for average speed? Can anyone explain to me why?


Thanks a lot.

How is the second question about average speed?

The question itself is a bit amibiguous, but assuming that you mean "how long will it take them to drive a total of 100 miles", you'd solve it the exact same way as you did the first example.

50m/8h = 25/4 mph

50m/6h = 25/3 mph

combined rate = 25/4 + 25/3 = 6.25 + 8.33 = approx 14.6mph total

100/14.6 = some ugly number under 7, but that's the answer.

Average speed questions are different that total speed questions.