DS - Help reqd - 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 - Help reqd

This topic has expert replies

Post new topic Post Reply

Previous Topic Next Topic

sankruth: Master | Next Rank: 500 Posts; Posts: 195; Joined: Sun Oct 21, 2007 4:33 am; Thanked: 10 times

DS - Help reqd

by sankruth » Sat Jan 12, 2008 10:13 am

k is a positive number. Is k a prime number?

1. No integer between 2 and SqRoot(k) inclusive, divides k evenly.
2. No integer between 2 and k/2 inclusive, divides k evenly, and k is greater than 5.

Explanations please!

Quote

Source: Beat The GMAT — Data Sufficiency | Sat Jan 12, 2008 10:13 am

StarDust845: Master | Next Rank: 500 Posts; Posts: 158; Joined: Mon Dec 03, 2007 8:32 am; Thanked: 7 times

by StarDust845 » Sat Jan 12, 2008 5:00 pm

Is the answer A?

Calista.

Quote

rchadha7: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Tue Oct 09, 2007 12:34 pm; Location: Jersey City

by rchadha7 » Sat Jan 12, 2008 6:57 pm

i think its D, whats the OA ?

just tried few nos. with condition 1 and everything is ok.

5,7,9,11,13,15,17,..... all non-prime are divided evenly by nos. less than sqrt k.

2. k/2 is greater than sqrt k for all k >2, thats why D

am i correct ?

Quote

simplyjat: Master | Next Rank: 500 Posts; Posts: 423; Joined: Thu Dec 27, 2007 1:29 am; Location: Hyderabad, India; Thanked: 36 times; Followed by:2 members; GMAT Score:770

by simplyjat » Sat Jan 12, 2008 9:21 pm

Both the statements alone are sufficient to answer the question.

During my computer programming classes I was told about these laws. If you want to find whether a number is prime or not, the optimal solution is to look till sqrt of that number.

The reasoning goes like this...
1. for 2 to n/2, if the number is indivisible, n/2 to n is 2 * ( 2 to n/2), thus we can ignore the second half as we have already tested 2...
2. for 2 to n/3, if the number is indivisible, n/3 to n is 3 * ( 2 to n/3), thus we can ignore the later as we have already tested 3...
And so on till be reach a number sqrt (n)....

I don't fully agree with the reasoning but it simply works