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gmat prep - Combinations

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by s_kaks » Thu Aug 21, 2008 9:22 am
one letter words can be chosen in 26c1 ways, ie, 26 ways
2 letter words can be chosen in 26c1*26c1 ways= 676 ways (since letters can be repeated)
3 letter words can be chosen in 26c1*26c1*26c1 = 17576 ways

Total = 26+676+17576 = 18278 ways
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by rishi235 » Thu Aug 21, 2008 9:29 am
Hey thanks man....

I must b asleep while solving this...... :roll:

I'd done it all right but goofed up the addition...... :(
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by lachlanc » Thu Aug 21, 2008 9:31 am
I worked it out by hand before realizing there is a short-cut. We know that any number with a units digit of 6 that is multiplied by itself will have a units digit of 6 also. 6x6 = 36, 16x16 = 256 etc.

Therefore, we know that 26 + 26^2 + 26^3 will add up to a number that has a units digit of 8 - 6+6+6=18. Since there is only answer with an 8 as a units digit, that the answer!
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