1-   49 50 = 100(X)
2-   49 50  > 100(O)  => C(2,2)  = 1x2 / 1x2
3-   48 49 50         => C(3,2)  = 2x3 / 1x2
4-   47 48 49 50      => C(4,2)  = 3x4 / 1x2
5-   46 47 48 49 50   => C(5,2)  = 4x5 / 1x2
....
24-  27 28   ... 50   => C(24,2) = 23x24 / 1x2
------------------------------
25 26 27    ...  50  =>  C(26,3) = 24x25x26 / 1x2x3 = 2600
-------------------------------
全加起來
=>  1x2 + 2x3 + 3x4 + 4x5 + ... + 23x24     / 1x2        + 2600
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套公式 [ 1x2 + 2x3 + ... + nx(n+1）= n(n+1)(n+2)/3  ]
n=23
=>  23x24x25 / 3 = 4600
--------------------------------------------------------------------
=>  4600 / 1x2    +2600 = 4900
=>  分子        =4900
    分母=C(50,3)=19600
               = 1/4