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jmc

number theory intermediate

Problem

The marching band has more than 100 members but fewer than 200 members. When they line up in rows of 4 there is one extra person; when they line up in rows of 5 there are two extra people; and when they line up in rows of 7 there are three extra people. How many members are in the marching band?
Solution
First, we look for an integer which leaves remainder of 1 when divided by 4 and a remainder of 2 when divided by 5. Checking the remainders of 2, 7, 12, 17, when divided by 4, we find that 17 is the least positive integer satisfying this condition. By the Chinese Remainder Theorem, the only positive integers which leave a remainder of 1 when divided by 4 and a remainder of 2 when divided by 5 are those that differ from 17 by a multiple of . Checking the remainders of 17, 37, when divided by 7, we find that leaves a remainder of 3. Again, using the Chinese Remainder Theorem, the integers which satisfy all three conditions are those that differ from 17 by a multiple of . Among the integers 17, 157, 297, , only is between 100 and 200.
Final answer
157