Question

When I run up a staircase, my stride can carry me up 1, 2, or 3 steps at a time. In how many ways can I run up a 9-step staircase (given that my last stride lands me on the 9th step)?

Answer 1

1+1+1+1+1+1+1+1+1
3+3+3
2+1+2+1+2+1
2+1+3+1+2