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Problem #332

Chopper

A very long stick is about to be passed through a special random chopper at speed 1\:m/s. Before passing the stick through the chopper, we put a special mark at some point on the stick. The special random chopper chops the stick at time t after the previous chop (or time t after starting for the first chop) with probability 5 \cdot e^{-5t}.

If the expected value of the length of the stick with the special mark is \frac{p}{q} in meters, where \gcd(p,q) = 1, find (p \cdot q^{-1})\:mod\:1000000007.

Contributed by Rudransh Aggarwal

Solved by 15 users

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