求助一道数学题

18. If d is positive integer, f is the product of the first 30 positive integers, what is the
value of d?
1). 10^d is a factor of f
2). d>6
Answer:
We have 5,10,15,20,25,30 and some 2s to see how many zeros there are in the tail of the
result 5x2, 15x2 contributes 1 zero each
10, 20, 30 => 3 zeros
25x4 => be careful, it contributes 2 zeros
So totally there are 1x2+3+2=7 zeros in the tails of 30!
For 1, d can be 1,2,3,4,5,6,7
For 2, d can be any integer >6
Together, d can only be 7 to fulfill both requirements.
Answer is C

我不明白,为什么不选E,15*8不就是120吗,为什么15*2呢?! 请NN指示.

可以的吧，无论15*8还是15*2都只产生一个零呀