Probability

A box contains coupons labelled $$1,2,3,\ldots,99, 100$$. Five coupons are picked at random one after another without replacement. Let the numbers on coupons be $$x_1,x_2,\ldots,x_5$$. The probability that $$x_1>x_2>x_3$$ and $$x_3<x_4<x_5$$ is $$\dfrac{a}{b}$$, where $$a$$ and $$b$$ are coprime positive integers. Find $$a+b$$.

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