A pair of fair dice is thrown twice.
So there is 36 * 36 = 1296 combinations.
The probability of getting 5, P(5) = { [(1,1), (1, 2)], [(1, 1), (2, 1)], [(1, 2), (2, 1)], [(2, 1), (1, 1)] }
= 
The probability of getting 11, P(11) = { [(1, 1), (2, 6)], [(1, 1), (6, 2)] [(1, 1), (4, 5)], [(1,1), (5, 4)]
[(1, 2), (3, 5)], [(1, 2), (4, 4)], [(1, 2), (5, 3)]
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For P(11) proceeding like the way, then get the result.