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If we list all the natural numbers below 10 that are multiples of 3 or
+ 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

Find the sum of all the multiples of 3 or 5 below 1000.
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(I generalized this to: 
   below 10**$n for code #1 and #2, 
or below $N for code #3)