1. The problem statement, all variables and given/known data In a particular atom, an alpha particle makes N collisions with a potential barrier in each second. The transmission coefficient at the barrier is 1.0e-15. In one second, 2.0e18 alpha particles are emitted from a group of 3.0e23 radioactive atoms. Find N. 2. Relevant equations ? 3. The attempt at a solution Frankly, I have no idea. I don't need a near-solution - just a hint as to where I should start from? I have figured out that total collisions in terms of N is 3.0e23N, but I've no idea what to do after that.