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Printjmc
number theory senior
Problem
Suppose that the least common multiple of the first positive integers is equal to . Find .
Solution
First, we observe that both and will divide into the least common multiple. Thus, will divide into the least common multiple, and so .
Also, we notice that and divide into the least common multiple. Thus, the sum of the digits must be divisible by : and the alternating sum of the digits must be divisible by (the divisibility rule for ): It follows that and . Summing the two equations yields that , of which only works. It follows that , and the answer is .
Also, we notice that and divide into the least common multiple. Thus, the sum of the digits must be divisible by : and the alternating sum of the digits must be divisible by (the divisibility rule for ): It follows that and . Summing the two equations yields that , of which only works. It follows that , and the answer is .
Final answer
740