DS - The Beat The GMAT Forum - Expert GMAT Help & MBA Admissions Advice

BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

DS

This topic has expert replies

Post new topic Post Reply

Previous Topic Next Topic

ketkoag: Legendary Member; Posts: 876; Joined: Thu Apr 10, 2008 8:14 am; Thanked: 13 times

DS

by ketkoag » Wed Apr 01, 2009 11:33 am

Joanna bought only $0.15 stamps and $0.29 stamps. How many $0.15 stamps did she buy?
(1) She bought $4.40 worth of stamps.
(2) She bought an equal number of $0.15 stamps and $0.29 stamps.

Why OA: A
My reason is : by statement 1 we have only one value i.e. 10 stamps of .15 as well as of .29. please lemme know whether there is any other possibility.

Quote

Source: Beat The GMAT — Data Sufficiency | Wed Apr 01, 2009 11:33 am

Jose Ferreira: GMAT Instructor; Posts: 99; Joined: Thu Mar 05, 2009 5:14 pm; Location: NYC; Thanked: 43 times; Followed by:9 members; GMAT Score:800

by Jose Ferreira » Wed Apr 01, 2009 7:09 pm

Hi,

You are right that the only combination of $0.15 and $0.29 that adds to $4.40 is 10*$0.15 + 10*$0.29 = $4.40. This is the reason that Statement 1 is sufficient.

Note that the only way A*$0.15 + B*$0.29 = $4.40 will work is if B is a multiple of 5:
A*$0.15 is definitely a multiple of 5. So is $4.40. Since we add B*$0.29 to one multiple of 5 (A*$0.15) and end up with another ($4.40), B*$0.29 must itself be a multiple of 5. This means B is a multiple of 5.

So we just need to check the cases where B = 0, 5, 10, 15.

If B = 0, A*$0.15 + 0*$0.29 = $4.40, or A*$0.15 = $4.40. $4.40 is not a multiple of $0.15, so this does not work.

If B = 5, A*$0.15 + 5*$0.29 = $4.40, or A*$0.15 = $2.95. $2.95 is not a multiple of $0.15, so this does not work.

If B = 10, A*$0.15 + 10*$0.29 = $4.40, or A*$0.15 = $1.50. $1.50 IS a multiple of $0.15, so this DOES work.

If B = 15, A*$0.15 + 15*$0.29 = $4.40, or A*$0.15 = $0.05. $0.05 is not a multiple of $0.15, so this does not work.

(We can check for divisibility by 15 by checking for divisibility by both 3 and 5. A number is divisible by 3 if its digits sum to a multiple of 3. A number is divisible by 5 if it ends in 0 or 5.)

Jose Ferreira
Founder and CEO, Knewton, Inc.
https://www.knewton.com/gmat

Quote

ketkoag: Legendary Member; Posts: 876; Joined: Thu Apr 10, 2008 8:14 am; Thanked: 13 times

by ketkoag » Wed Apr 01, 2009 10:09 pm

Thanks a lot for a nice explanation Jose!
Also, is it possible for us to chat online sometime so that i can clear some general querries regarding the GMAT?