Please help

Alan and Brian are competing in solving Rubik's cubes. They are using an identical set of prepared Rubik's cubes [i.e. Alan and Brian's cube have the same starting pattern in each round). Once one of them solves a cube, the other one has to stop and move on to the next cube so that they are always solving the same cube. On average, Alan needs 30 seconds to solve a cube while Brian needs 20 seconds. Suppose the time they actually need to solve each of the cubes follows independent exponential distributions (independent of each other, also independent on each cube). Suppose Brian solves N cubes before Alan solves one successfully and suppose it takes Alan T seconds until he successfully solves his rst cube. The distribution of T|N : 4 is