For the first number out of 16 there is a choice of 16; for the second there are 15 remaining; then 14, and so on. In total then there are 16*15*14*13*12 but there are 120 ways of arranging these numbers, so as we are only interested in combinations where order does not matter, we need to divide by 120. That gives us 4,368 rows of 5 columns with no repeating sets.