Which one of the following numbers is the greatest positive integer x such that 4^x is a factor of 64^5?
Choose the correct choice from the following:
A) 5
B) 8
C) 10
D) 15
E) 19
64 = 4*4*4= 4^3   64^5 =(4^3)^5   =4^15   we have to find the greatest positive integer value for x   when x=15, it divides exactly once and that is the least positive integer value of the quotient   therefore, the correct option is 15
<result>D</result>
Which one of the following numbers is the greatest positive integer x such that 4^x is a factor of 64^5?