If n is the product of the integers from 1 to 20 inclusive, what is the greatest integer k for which 2k is a factor of n?
10
12
15
18
20
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即n里面有多少个2 相乘。
2,4,6,8,10,12,14,16,18,20
共:1+2+1+3+1+2+1+4+1+2=18
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