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Printjmc
number theory intermediate
Problem
What is the largest positive integer with only even digits that is less than and is a multiple of ?
Solution
We can tell that the integer is a multiple of if the sum of its digits is a multiple of . For the largest integer with even digits that is less than , it must have the largest even digit, , in the thousands place. So we have . Notice that the maximum integer with even digits is . However, the digits must add up to a multiple of , and more specifically, an even multiple of since all of the digits are even. The closest even multiples of are and , but the sum of the maximum integer is only . So the sum of the digits must be . We maximize the integer with , with a sum of , which leaves a sum of for the remaining digits. For the digits to be even and to maximize the integer, the tens digit must be and the units digit must be . The largest integer is .
Final answer
8820