What replaces the Question Mark

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Answer this Question Correctly (#Brain Challenging#)
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1,1,2,3,8,?
What replaces the Question Mark "?"......

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The Answer is 15. Below is the logic
This is a series of Double Factorial.
Rule of Double Factorial.
If n is even, then
n !! = n ( n - 2)( n - 4)( n - 6) ... (4)(2)
If p is odd, then
p !! = p ( p - 2)( p - 4)( p - 6) ... (3)(1)
So, accrding to the above logic..
0!!=1
1!!=1
2!!=2
3!!=3
4!!=4*(4-2)=4*2=8
The value u hav to find is...
5!!=5*(5-2)*(5-4)=5*3*1=15.
So, the ansr is 15.