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number theory intermediate

Problem

A group of students, where , is on a field trip. If their teacher puts them in groups of 8, the last group has 5 students. If their teacher instead puts them in groups of 6, the last group has 3 students. What is the sum of all possible values of ?
Solution
We are given that and . We begin checking numbers which are 5 more than a multiple of 8, and we find that 5 and 13 are not 3 more than a multiple of 6, but 21 is 3 more than a multiple of 6. Thus 21 is one possible value of . By the Chinese Remainder Theorem, the integers satisfying and are those of the form , where is an integer. Thus the 2 solutions less than are 21 and , and their sum is .
Final answer
66