For a positive integer n, if 5^n is a factor of 20!, but 5^n+1 is not a factor of 20!, what is the value of n?	Choose one
A) 4 B) 6 C) 7 D) 8 E) 9
20! has four 5's     20/5= 
<gadget id="calculator">  20/5</gadget>
<output>4</output>
4   Thus 5^4 can completely divide 20! and 5^5 can not divide 20!   So, 4nswer will be (4) 4
<result>A</result>
For a positive integer n, if 5^n is a factor of 20!, but 5^n+1 is not a factor of 20!, what is the value of n?