Start with a number and repeat: if is even, halve it; if is odd, triple it and add one. For example, starting with 3 gives 3, 10, 5, 16, 8, 4, 2, 1. Rearrange the numbers in ascending order and mark those places across row so that the third row has marks at the places 1, 2, 3, 4, 5, 10, 16.