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jmc

counting and probability senior

Problem

Define an ordered quadruple of integers as interesting if , and . How many interesting ordered quadruples are there?
Solution
Rearranging the inequality we get . Let , then is a partition of 11 into 5 positive integers or equivalently: is a partition of 6 into 5 non-negative integer parts. Via a standard stars and bars argument, the number of ways to partition 6 into 5 non-negative parts is . The interesting quadruples correspond to partitions where the second number is less than the fourth. By symmetry, there are as many partitions where the fourth is less than the second. So, if is the number of partitions where the second element is equal to the fourth, our answer is . We find as a sum of 4 cases: two parts equal to zero, ways, two parts equal to one, ways, two parts equal to two, ways, two parts equal to three, way. Therefore, and our answer is .
Final answer
80