Find the smallest such that ! ends in 290 zeroes.
Solution
Each 0 represents a factor of . Thus, we wish to find the smallest factorial that contains at least 290 2's and 290 5's in its prime factorization. Let this number be !, so the factorization of ! contains 2 to the power and 5 to the power , where (this takes into account one factor for each single multiple of 2 or 5 that is , an additional factor for each multiple of or , and so on). Naturally, because 2 is smaller than 5. Thus, we want to bring as low to 290 as possible. If , we form a rough geometric sequence (by taking away the floor function) whose sum is represented by . Hence we estimate , and this gives us . Adding 10 to the value of gives the necessary two additional factors of 5, and so the answer is 1170.
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