Question 21
A flight of stairs has 11 steps. David can go up 1 step, 2 steps, or 3 steps at a time. The 6th
step and the 9th step cannot be stepped on as they are destroyed. How many ways are
there for David to go up the stairs?
step and the 9th step cannot be stepped on as they are destroyed. How many ways are
there for David to go up the stairs?