GMAT Prep %

moneyman · Posted: Sat May 10, 2008 6:18 am Post subject: GMAT Prep %

Should I pick numbers for this question ? What is the best strategy ?

During a trip on an expressway, Don drove a total of x miles. His average speed on a certain 5 mile section of the expressway was 30 miles an hour , and his average speed for the remainder of the trip was 60 miles per hour. His travel time for the x mile trip was what percent greater than it would have been if he had travelled at a constant rate of 60 miles per hour for the entire trip ?

8.5%
50%
x/12 %
60/x %
500/x %

Ans E

Diatance = Time*Rate

For the 5 mile section Time=1/6 or 10 mins

For the remainder Distance= (x-5) Rate=60 So Time = (x-5)/60 or (x-5)mins

Total time = (x-5)mins + 10 mins

Option 2

Distance = x Rate=60 so Time = x/60 or x mins

This comes down to how much more is x+5 > x ?

I know I am missing something here. PLease help!! Exam is on Monday

VP_Jim · Posted: Sat May 10, 2008 5:48 pm Post subject:

Hi moneyman,

An easier way to look at this problem is to plug in numbers, so how about using 20 for X? With X = 20, Don took 10 minutes if he traveled 5 miles of the total distance at 30 miles/hour, plus 15 minutes for the rest of the trip at 60 miles/hour, for a total of 25 minutes.

However, if he traveled the entire distance at 60 miles/hour, it would only have taken him 20 minutes.

Thus, 25 minutes - 20 minutes = a difference of 5 minutes, and 5/20 = 25% (the difference between the two options). Now, find the answer that gives you this percentage, by plugging in the original number you chose for X (which is 20) and you'll see that the correct answer is choice E.

One last word of advice, no matter how good you are at algebra, you're better at arithmetic; so plug in whenever possible.

Good luck on your exam!

moneyman · Posted: Sat May 10, 2008 9:16 pm Post subject:

Thanks a lot Jim..I am very confortable with using numbers but I thought I could save some time by using this approach..but guess it dint work quite well..

CITI29 · Posted: Sun May 11, 2008 9:24 pm Post subject:

Time taken with two seperate speeds:
5/30 +x-5/60 =x+5/60

Time if the speed 60miles/hr was constant:
x/60

Difference in two times : x+5/60 -x/60 =5/60

%age (5/60) / x/60 *100
=500/x

[E]