2 dice are rolled 1 black and 1 white How many rolls occur

2 dice are rolled , 1 black and 1 white

How many rolls occur in which the value on the white die is greater than the value on the black die?

Consideing all the possibilities of (W,B) to occur, only following outcomes are favourable :

For result below (i,j) means i belongs to white dice & j to black one.

(2,1)
(3,1),(3,2)
(4,1),(4,2),(4,3)
(5,1),(5,2),(5,3),(5,4)
(6,1),(6,2),(6,3),(6,4)(6,5)

Total favourable outcomes : 1+2+3+4+5 =15