Arithmetic

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Mon Dec 06, 2010 3:23 pm

??

Thanks for your help

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Source: Beat The GMAT — Problem Solving | Mon Dec 06, 2010 3:23 pm

N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Mon Dec 06, 2010 3:29 pm

Ignore the 2nd q, he says the smallest 2 prime no.s > 50, so I thought it's 2 53s, but he wants 53+59= 112

I didn't know we should memorize beyond 50 for prime no.s

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uwhusky: Legendary Member; Posts: 1172; Joined: Wed Apr 28, 2010 6:20 pmFollowed by: 4 members; Thanked: 74 times

by uwhusky » Mon Dec 06, 2010 5:00 pm

Question 1 is essentially asking how many odd numbers there are between 0 and 10: 5.

Question 2 is testing whether you understand what prime numbers are. You don't need to memorize them, but you should know pretty quickly that any even number > 2 = not prime, and adding any two numbers together = multiple of 3's = not prime. Multiple of 4's follow the same rule as 2's. Multiple of 5's are numbers that end in 0 or 5. Multiple of 6's follow the same rule as 3's. 7 is harder, but GMAT won't ask you to figure out a whether a large non-obvious number is prime.

The key for #2 is to memorize the methods of establishing whether a number is multiples of numbers between 2 to 9.

Last edited by uwhusky on Mon Dec 06, 2010 5:19 pm, edited 1 time in total.

Yep.

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Mon Dec 06, 2010 5:17 pm

Can you explain what's meant by remainder of 1? I don't get it

nor this "adding any two numbers together = multiple of 3's = not prime"

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uwhusky: Legendary Member; Posts: 1172; Joined: Wed Apr 28, 2010 6:20 pmFollowed by: 4 members; Thanked: 74 times

by uwhusky » Mon Dec 06, 2010 5:22 pm

Remainder: https://en.wikipedia.org/wiki/Remainder

So any odd number divided by 2 will have 1 as the remainder.

24: 2 + 4 = 6. 6 is a multiple of 3, and therefore 24 is a multiple of 3.

126: 1 + 2 + 6 = 9. 9 is a multiple of 3, and therefore 126 is a multiple of 3.

3257757 = 3 + 2 + 5 + 7 + 7 + 5 + 7 = 36. 36 is a multiple of 3, and so forth...

I strongly recommend you to pick a MGMAT Number Property book, and go through it very very thoroughly.

Yep.

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N:Dure: Master | Next Rank: 500 Posts; Posts: 181; Joined: Sat Oct 16, 2010 1:57 pm; Thanked: 4 times

by N:Dure » Mon Dec 06, 2010 5:49 pm

Perfect! Thanks for explaining, especially the multiple part.

Btw these qs are from GRE not GMAT, and this was the 1st q on an actual CAT. So you might notice they are way easier than the GMAT.

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