Math questions from mba.com and GMAT Prep software
tnsomu
 
 

Math - What's the quick way to solve this

by tnsomu Sun Oct 05, 2008 5:45 pm

How many seconds will it take for a car that is traveling at a constant rate of 45 miles per
hour to travel a distance of 22 yards? (1 mile = 1,160 yards)
A. 8
B. 9
C. 10
D. 11
E. 12
so19
 
 

by so19 Sun Oct 05, 2008 10:02 pm

1 mile = 1,760 yards...
RonPurewal
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Posts: 19747
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by RonPurewal Fri Oct 24, 2008 4:40 am

http://s3.supload.com/free/conversion_p ... .jpg/view/

the link shows the work for using CONVERSION FACTORS to accomplish this conversion.
here are the basics of the process:

* start with the starting information, usually a concrete value. here, that concrete value is 22 yards.
note that rates are not "starting information"; those count as conversion factors.

* to accomplish the conversion, create CONVERSION FACTORS, which have EQUAL quantities on the top and bottom of the factor. there are two types of such conversion factors:
- RATES (which are naturally fractions already)
- REFERENCE CONVERSIONS (such as 12 inches = 1 foot) which are universally true and could be looked up in a reference book if necessary.

* position the fractions so that UNWANTED UNITS CANCEL and the desired units are left in the desired places.
- note how the 45 mph is positioned so that it's 1 hour per 45 miles, not 45 miles per 1 hour. this is done so that the miles are cancelled out.

--

in the problem shown, 22/1760 cancels down to 1/80, and 3600/45 cancels down to 80/1. therefore, 80/80 = 1, so the whole fraction gives 1 second. are you sure you didn't copy some number with an incorrect decimal place or other similar error?

plus, think about this intuitively: if a car is traveling at 45 miles per hour, which is approaching highway speeds, then it's ridiculous to think that it would take 8 to 12 seconds to travel 22 yards. the average human can run 22 yards in about three seconds. check the problem!
PFWinkler
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Re: Math - What's the quick way to solve this

by PFWinkler Tue May 24, 2016 8:28 pm

Yes - There is a mistake here. It should be 220 yards....

Source is GMAT Prep
RonPurewal
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Posts: 19747
Joined: Tue Aug 14, 2007 8:23 am
 

Re: Math - What's the quick way to solve this

by RonPurewal Wed May 25, 2016 2:26 am

thanks!