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jmc

counting and probability senior

Problem

Camy made a list of every possible distinct five-digit positive integer that can be formed using each of the digits 1, 3, 4, 5 and 9 exactly once in each integer. What is the sum of the integers on Camy's list?
Solution
Note that there are numbers ending in 1, since we have 4 choices for the 10s digit, 3 choices for the 100s digit, 2 choices for the 1000s digit, and 1 choice for the remaining digit. Thus there are also 24 numbers ending in each of 3, 4, 5, 9, and the total contribution of ones digits to the sum is . But we can make a similar argument about the contribution of the digits in the other places (10s, 100s, etc.), so our total sum is .
Final answer
5,\!866,\!608