Problem:
Let be the maximum number obtainable as a product of positive integers, all distinct, whose sum is . With how many digits "0" does the decimal representation of end?
Problem:
Let be the maximum number obtainable as a product of positive integers, all distinct, whose sum is . With how many digits "0" does the decimal representation of end?
Pick one
Solution:
The answer is (D). I call the set of numbers that sum to and whose product is given by the maximum possible value, that is . I call "hole" a number that is not in , but such that contains both a number greater than and a number smaller than .
I prove that does not contain : let be the maximum integer contained in ; if contained , then the set obtained from by removing and adding would still respect all the conditions imposed by the problem, but the product of the numbers contained in it would be , contradicting the fact that was the maximum possible.
I prove by contradiction that has at most one hole: suppose that , with and such that and are two distinct holes; since , the set obtained from by removing and adding and still respects all the hypotheses of the problem. Moreover, the product of the numbers contained in would be given by , contradicting the maximality of .
Furthermore, cannot fail to contain while at the same time having a hole such that . Indeed, if this were the case, the set , obtained from by adding and and removing , still respects the hypotheses of the problem. Once again, the product of the numbers contained in is given by , which implies that was not the maximum possible.
The only way to write as a sum of distinct numbers respecting the previous conditions is by using the numbers from to , skipping . It follows that , which ends with digits '0'. To count the number of trailing zero digits it is enough to count the number of factors and the number of factors that appear in the factorization of , and then take the minimum of these two quantities. It is easy to verify that there are fewer factors than factors ; finally, to compute the number of factors , one can observe that these are the same as those of the product : this product has factors (indeed every multiple of between and contains only one factor , except for and which contain each).