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How many integers?

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How many integers?

by Vemuri » Tue Mar 17, 2009 6:22 pm
How many integers between 0 and 1570 have a prime tens digit and a prime units digit?

A. 295
B. 252
C. 236
D. 96
E. 76

Is there no easy way to attempt these type of problems? I sometimes find the less lengthy questions more time consuming than the long story telling questions.
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by cramya » Tue Mar 17, 2009 7:19 pm
tens and units must be 2,3,5 or 7


I would go with B 252

2 digit numbers = 4*4 = 16

3 digit numbers = 9*4*4 = 144

4 digit number

first digit has to be 1 i.e 1 choice
second digit can be 0,1,2,3,4,5 i.e 6 choices
3 rd digit can be 2,3,5,7 -> 4 choices
4th digit can be 2,3,5,7->4 choices

6*4*4 = 96

16+144+96 = 256


But we would have included 1572,1573,1575,1577 in this

So 256-4 = 252
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by Vemuri » Tue Mar 17, 2009 11:42 pm
perfecto !!! I like your approach. :)
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