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
Arithmetic
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uwhusky
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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.
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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uwhusky
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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.
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.
