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 Post subject: Arithmetic - page 52 problem #36Posted: Mon Feb 06, 2012 6:51 pm

Joined: Mon Feb 07, 2011 8:58 pm
Posts: 1
Quick question regarding this problem. Would you advise listing out all the numbers between 50 and 70 and writing out next to them if they are prime? (Like they did in the answer guide) I was able to eliminate most by taking out those divisible by two and five but always ended up listing them all out just because having all them in front of me is easier to remember.

Is there some kind of short cut I may be missing or is listing each number between 50-70 out the best way? Hope that made sense. Thanks for the help!

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 Post subject: Re: Arithmetic - page 52 problem #36Posted: Tue Feb 07, 2012 4:24 pm

Joined: Thu Feb 12, 2009 6:32 pm
Posts: 497
Here is the best approach:

list out all numbers between 50 and 70, but don't list evens and numbers ending in 5 since they are obviously not prime:

51
53
57
59
61
63
67
69

eliminate numbers divisible by 3:

53
59
61
67

check for divisibility by 7

(none are)

53
59
61
67

(don't need to check 11, because 7*11 = 77, which is greater than our range)

- Veritas Help

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