Okay so take this solution: "A recursive approach is quick and easy. The number of ways to climb one stair is 1. There are 2 ways to climb two stairs: 1,1 or 2. For 3 stairs, there are 4 ways: (1,1,1) (1,2) (2,1) (3)
For four stairs, consider what step they came from to land on the fourth stair. They could have hopped straight from the 1st, done a double from #2, or used a single step from #3. The ways to get to each of these steps are 1+2+4=7 ways to get to step 4. The pattern can then be extended: 4 steps: 1+2+4=7 ways. 5 steps: 2+4+7=13 ways. 6 steps: 4+7+13=24 ways." Okay, so you got that solution, now what formula is that? Please I need this by 8:00 pm.