Question

Number of ways in which a person can walk up a stairway which has 7 steps if he can take 1 or 2 steps up the stair at a time, is

• A. 28
• B. 21
• C. 15
• D. 17

Answer 1

Answer: (B)

21

Total steps = 7

No. of Steps size 1	No. of steps of 2	No. of ways
7	0	$1$
5	1	$\frac{6!}{5!}=6$
3	2	$\frac{5!}{3!2!}=10$
1	3	$\frac{4!}{3!1!}=4$
	Total	$21$