How many alpha particles rebound?

In one gold foil experiment, about $1$ alpha particle in $8000$ bounces straight back. You will make the program work out the rebounds for several numbers of particles, draw them as bars, and find how many particles must be fired to expect a set number of rebounds.

The idea. The rebounds are a fixed share of the particles fired, so divide the number fired by $8000$. Going the other way, multiply.

Your tasks.

1. In rebounds, return the expected number of rebounds out of fired particles.
2. Fill the list counts with the rebounds for every number in totals.
3. Find needed, the number of particles to fire to expect $25$ rebounds.