To form a zero we need one 2 and one 5. In general any factorial contains more 2's than 5's. So limiting factor is 5's. We will find how many 5's each factorial has.

2! and 4! does not contain any zeros. 8!, 9! contains 1 and 10! and 11! contains 2 each.

So total zeroes are 1 x 8! + 1 x 9! + 2 x 10! + 2 x 11! = 8!+9!+2(10!)+2(11!)

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