Answers

2014-10-03T21:37:30+08:00
The answer is 840. I added 15+15, and divided it by 2,5,7,8, and 15 every time. Don't believe me? Try it for yourself.
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2014-10-03T23:58:34+08:00
The smallest counting number divisible exactly by  2578 & 15 is the
 LCM  of the two numbers
prime factors of  2578    is      2, 1289,
prime factors of  15        is       3, 5,
since there are no common factors between the two numbers
the  LCM       is      2578* 15  =  38,670
so
       38,670 is the smallest number that is divisible exactly by
         2578 &  15


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