In each of the following question, there is a certain relationship between two given number on one side of (::) and one number is given on the other side of (::) while another number is to be found from the given alternative having the same relationship with this number as the number of the given pair. Choose the best alternative.
63 : 21 :: 27 : ?
1) 6
2) 9
3) 1
4) 3

Explanation

4,