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Where rolling a pair of die, a pair is two so there is two die. Now we have to look at the sample space for the two dice.
1 2 3 4 5 6
1 l 2l 3l 4l 5l 6l 7l
2l 3l 4l 5l 6l 7l 8l
3l 4l 5l 6l 7l 8l 9l
4l 5l 6l 7l 8l 9l 10l
5l 6l 7l 8l 9l 10l 11l
6l 7l 8l 9l 10l 11l 12l
there is 36 outcomes in the sample space none of them include getting 15.