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mehrasa: Master | Next Rank: 500 Posts; Posts: 279; Joined: Fri Nov 05, 2010 5:43 pm; Thanked: 15 times; Followed by:1 members

OG_ # 87

by mehrasa » Tue Sep 06, 2011 11:54 pm

Is the number of seconds required to travel d1 feet at
r1 feet per second greater than the number of seconds
required to travel d2 feet at r2 feet per second?
(1) d1 is 30 greater than d2.
(2) r1 is 30 greater than r2.

to me, the correct answer is "c" since we will have d2+30/r2+30 compared with d2/r2
which the former fraction is bigger than the latter then the answer is "together sufficient"
However, OA is E[/quote]

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Source: Beat The GMAT — Data Sufficiency | Tue Sep 06, 2011 11:54 pm

bblast: Legendary Member; Posts: 1077; Joined: Mon Dec 13, 2010 1:44 am; Thanked: 118 times; Followed by:33 members; GMAT Score:710

by bblast » Wed Sep 07, 2011 2:59 am

Hi, this question is not really testing RTD, but its testing how fractions vary when the same number is added to the numerator and the denominator.

remember 2 rules :

1>If the same integer is added to the num and den of a proper fraction then the value of fraction increases.
eg 1/3, add 1 to both N and D we get 2/4 which is greater than 1/3

2>The converse is true for a positive improper fraction.
eg 4/3(1.33), add 1 to both N and D we get 5/4(1.25) which is less than 4/3.

This question does tell us that values are positive but does not tell us whether the fraction d1/r1 is proper or not. So the statement will not hold if the fraction is improper.

Cheers !!

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tpr-becky: GMAT Instructor; Posts: 509; Joined: Wed Apr 21, 2010 1:08 pm; Location: Irvine, CA; Thanked: 199 times; Followed by:85 members; GMAT Score:750

by tpr-becky » Wed Sep 07, 2011 3:03 am

This is a rate problem so start with the basic formula d1 = r1(t1) and d2=r2(t2)

The question asks if t1>t2 - you can reorganize the formula to restate is d1/(r1)>d2/(r2)

1) by itself this is clearly not enough becuase we have no rate information - eliminate AD

2) by itself this is not enough becuase we don't have distance information - eliminate B

Together the statement are still not enough. basically together we have the question:
is (d2 + 30)/(r2 + 30) > d2/(r2) When you have division and addition together with variables it is rarely enough because the relationship will be dependent on values, it is best to try some values:

For instance if d2 = 10 and r2 = 50 the question is is 40/80 > 10/50 which reduces to is 1/2 > 1/5 (yes)

But if d2 is 10 and r2 = 10 then the question is is 40/40 > 10/10 - which asks is 1 > 1 - (no)

Therefore together the information is insufficient.

Becky
Master GMAT Instructor
The Princeton Review
Irvine, CA