See the image on the right.
The 6 red branches depict the outcomes of the first roll.
The 6 green branches depict the outcome of the 2nd roll, for each of the outcome of the first roll.
Draw such "6 green branches"for 3,4,5,6 (outcomes of first roll) as well.
count all the green and red branches.
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"You don't have to believe in God, but you should believe in The Book." -Paul Erdős