how many different two-digit numbers greater than 50 can be formed from the digits 3,6,7, and 9 of repetition of digits is allowed?

Respuesta :

First of all  the  repeated digits numbers are  66,77 and 99   Total 3
Numbers beginning with 6  Total 3  (63,67 and 69).
Numbers begin with 7   Total3
and begin with 9         Total 3.

answer is 12