integer

This topic has expert replies
Junior | Next Rank: 30 Posts
Posts: 19
Joined: Thu Dec 07, 2006 3:20 am

integer

by Chrystelle » Fri Jan 26, 2007 7:47 am
For every positive even integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) + 1, then p is

a) between 2 and 10
b) between 10 and 20
c) between 20 and 30
d) between 30 and 40
e) greater than 40
Source: — Problem Solving |

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2228
Joined: Wed Dec 27, 2006 3:28 pm
Location: Montreal, Canada
Thanked: 639 times
Followed by:694 members
GMAT Score:780

by Stacey Koprince » Fri Jan 26, 2007 11:51 pm
Hi, Chrystelle - please be sure to post the source for your question. Thanks!
Please note: I do not use the Private Messaging system! I will not see any PMs that you send to me!!

Stacey Koprince
GMAT Instructor
Director of Online Community
Manhattan GMAT

Contributor to Beat The GMAT!

Learn more about me

User avatar
Community Manager
Posts: 789
Joined: Sun Jan 28, 2007 3:51 pm
Location: Silicon valley, California
Thanked: 30 times
Followed by:1 members

by jayhawk2001 » Sun Jan 28, 2007 4:03 pm
h(100) + 1
= 2 * 4 * ... * 100 + 1
= 2*50! + 1

2*50! can be expressed as a multiple of X (2 <= X <= 50) and so
(2*50! + 1) cannot be a multiple of of that same number X.

Since p is a factor of h(100) +1, p should be > X and so option
"E" looks like the correct answer.

Thoughts ?

User avatar
Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Fri Nov 26, 2010 12:13 pm

by savidh » Fri Nov 26, 2010 12:58 pm
this is an official gmat queionst and the answer is indeed E. i still haven't figured out why though.

User avatar
Master | Next Rank: 500 Posts
Posts: 332
Joined: Tue Feb 09, 2010 3:50 pm
Thanked: 41 times
Followed by:7 members
GMAT Score:720

by rishab1988 » Fri Nov 26, 2010 2:37 pm
We know that h(100) contains the product of even integers.

h(100) -> 100.98.96.....2.

We know that h(100) is divisible by 2.because it has a 2 in it.But if we add 1 to a 2 we will not get to the next multiple of 2.Eg lets say h(100)= 222.Then adding1 to it will give us 223,which is not a multiple of 2

Same case for 3.We know h(100) is divisible by 3 because it has 6.But if we add 1 to a multiple of 3 we will not get to the next multiple of 3.

Similarly h(100) is a multiple of 5 for it has 10. same for other prime number 7(14),11 (22),13(26),17(34),19(38),23(46),29(58),31(62),37(74),41(82).In this way I have eliminated all answer choices except E.

Therefore,the answer is E.

Basically the concept tested by this questions is - if you add a multiple of a number say X to another multiple of the same number X,you will get another multiple of X.

For eg : if you have a multiple of a number say (7) -> 84.If you add another multiple of this number say 70 to 84.The resulting number (154) too will be a multiple of 7.

Hope this helps.