using factorial in probability question

[resolved]

Thanks chromes,

I tried your solution in HOME mode and got the correct solution
I tried your solution in CAS mode and the answer was undef ???? why does this solution only work in the one mode????

Sad that the Prime can not handle the raw numbers.....the TI-85 can, the Prime's failure has cost me three hours of frustration trying to figure out what I was doing wrong...