jsphcal wrote:

Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip and then at an average speed of 60 miles per hour for the remaining 30 miles of the trip. If she made no stops during the trip, what was Susan's average speed, in miles per hour, for the entire trip?

a. 35

b. 40

c. 45

d. 50

e. 55

The Common Formula is Average speed = Total distance / Total time.

However if the distances traveled with two different speeds are the same (as 30 in this case), we can calculate the average speed as per following rule.

ratio of two distinct speeds = 1:2

Difference of two distinct speeds = 60 - 30 = 30

Now divide this difference by the sum of intergers of ratio (i.e. by 1+2 = 3 in this case ) 30/3 = 10

Average speed will be 10 X 1 part away from the lower speed i.e. 30 + 10 X 1 = 40.

The benefit of this method is once practiced enough you can calculate the average speed mentally or with minimal calculation and thereby can save valuable time on the GMAT

Regards,

Abhijit

_________________

Live Q&A with B-School Adcoms - 2015 Season

Be the coolest guy in the MBA Forum - Be a threadmaster!

Have a blog? Feature it on GMAT Club!

All MBA Resources All 'Sticky' Topics at one place

Please share your profiles for this application season: 2015 Profiles w/ Admit/Dings Results!

GMAT Club Premium Membership - big benefits and savings

Next Generation GMATClub CATS with Brilliant Analytics.