Is there any better approach to solve this Simple Question?

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gmataspirant: Senior | Next Rank: 100 Posts; Posts: 52; Joined: Mon Jan 14, 2008 7:46 pm; Thanked: 3 times

by gmataspirant » Sun Jun 01, 2008 5:47 pm

Hey Guys,

Is there any short cut method availalbe to answer the following simple question ?

What is the greatest integer that is a sum of four different prime numbers, each less than 30 ?

A 67
B 88
C 98
D 104
E 126

I narrowed down the options to A,B,C,D and ignored option E as it is over 120. Answered the question by summing up different prime numbers which are less than 30. I am sure that there must be a smarter approach to solve this question.

Never GiveUp For Any Reason

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Source: Beat The GMAT — Problem Solving | Sun Jun 01, 2008 5:47 pm

jasonc: Master | Next Rank: 500 Posts; Posts: 127; Joined: Sat Apr 19, 2008 1:02 am; Thanked: 12 times

by jasonc » Sun Jun 01, 2008 6:18 pm

not sure how to do it faster... but even if you just sum up the 4 primes, it shouldn't take more than 1 min.

count down from 30 to find the highest 4 primes, sum those = answer.
29, 23, 19, 17=88

I beat the GMAT! 760 (Q49/V44)

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sasi78: Junior | Next Rank: 30 Posts; Posts: 14; Joined: Fri Apr 04, 2008 11:28 pm

by sasi78 » Sun Jun 01, 2008 7:07 pm

I think, no need to use POE for this kind of problems. Solving directly is the simple & less time consuming way.

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ksh: Master | Next Rank: 500 Posts; Posts: 106; Joined: Sun Feb 17, 2008 10:09 pm; Thanked: 3 times

by ksh » Mon Jun 02, 2008 3:39 am

I think you have solved the right way.

It is difficult to deduce any formulae for it. Its better to count n add 29+23+19+17=88.

Cheer up

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VP_RedSoxFan: GMAT Instructor; Posts: 85; Joined: Thu May 01, 2008 12:56 pm; Location: Salt Lake City, UT; Thanked: 24 times; GMAT Score:750+

by VP_RedSoxFan » Mon Jun 02, 2008 6:35 am

Sometimes we can out-think ourselves when trying to come up with a neat "trick". As has been mentioned, take the 45 seconds or so and focus energies on getting the 4 greatest prime numbers less than 30 accurately.

Ryan S.
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Elite GMAT Preparation and Admissions Consulting
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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Mon Jun 02, 2008 4:01 pm

There's no formula for finding primes; the only reliable way to do this problem is to locate the four largest primes less than 30 and add them up. You really should know all primes up to at least 40, and if you do, this question should only take 30 seconds- you only need to add four small numbers. Otherwise, it still should only take a minute.

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