You walk up 12 steps going up either 1 or 2 levels with each

You walk up 12 steps, going up either 1 or 2 levels with each stride. There is a snake on the 8th step, so you cannot step there. How many ways can you go up? For example, one way is 1-1-1-1-1-1-1-2-1-1-1, while another is 2-2-2-1-2-2-1.

Solution

one way is we go 1 step each till 7th that is 7C7 = 1

2nd is we go 1-3-5-7 alternate steps till 7th that is 4C3 = 4

another is we take either 1 step then 2 or first 2 then 1 so it will cover 6 steps and then 1 last 7th so that is = 6C1 = 6

and one more is that we take first 5 steps or last five steps and remaining two one by one So that is 5C2 = 10

So total is 1+4+6+10 = 21

but

also it says

each stride will cover 1 or 2 levels

it means answer times 3 strides

so

3*21 = 63