Counting with combinations

You will write three functions that find probabilities for draws from a group, using math.comb: the number of selections, the chance of exactly $k$ special objects, and the chance of at least $k$. The checker compares them with a count of every selection in a small case.

The idea. A group has special objects of one kind and others of another. A selection of drawn objects has exactly k special ones in $^{\text{special}}C_k \times {}^{\text{others}}C_{\text{drawn}-k}$ ways. All selections number $^{\text{special}+\text{others}}C_{\text{drawn}}$.

Your tasks.

1. Finish selections(n, r). Return the number of ways to choose r objects from n.
2. Finish chance_exactly(special, others, drawn, k). Return the probability of exactly k special objects as a Fraction.
3. Finish chance_at_least(special, others, drawn, k). Add the chances of k, k + 1, and so on.