A band has 72 members who will all be marching during halftime. They need to march in rows with the same number of students per row. If there must be between 5 and 20 students per row, in how many possible row-lengths can the band be arranged?
\(72 = 2^3 3^2 \\ \text{This gets us, given the constraints on row length,}\\ 6\times 12,~8\times 9,~9\times 8,~12\times 6\\ \text{so 4 different row lengths}\)
.