Posted by jen on Monday, September 27, 2010 at 9:30pm.
An elf has a staircase with 10 steps to climb. Each step it takes can cover either one stair or two stairs. In how many ways can the elf climb the stairs?

math  Reiny, Monday, September 27, 2010 at 10:14pm
5 Doubles === 1 way
4 Doubles, 2 Singles === e.g. DDSSDD
number of ways = 6!/(2!4!) or C(6,4) = 15
3 Doubles, 4 Singles === C(7,3) = 35
2 Doubles , 6 Singles === C(8,2) = 28
1 Double , 8 Singles === C(9,1) = 9
10 Singles === 1
total = 89

math  huh, Tuesday, September 28, 2010 at 6:57pm
...what?

math  karina, Sunday, October 3, 2010 at 5:27pm
i don't understand how its 89 ways!
how about 20 stairs, and N stairs? what would be the equation for that?
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