There are 40 balls in a box. 4 are white and rest 36 are black. If one randomly pick 20 balls from the box, what is the probability of getting 1 white ball? 2 white balls? 3 white balls? 4 white balls?
Also, I repeat this experiment n times to collect total 100 picks of white balls. How do one calculate this n ?
The chance of getting $1$ is $$\frac {{4 \choose 1}{36 \choose 19}}{40 \choose 20}=\frac {120}{481}$$ The chance of getting $3$ is equal to the chance of getting $1$. If you draw and get $1$ the remaining balls are a draw that would give you $3$.