The failure table

A dictionary holds the typical use failure rates of some methods, as pregnancies in the first year among $100$ users. You will find the pregnancies expected in a group, the chance over several years, and the method with the lowest rate.

The idea. In a group of $n$ couples, about $\dfrac{n \times \text{rate}}{100}$ pregnancies are expected in a year. The chance of at least one pregnancy in $t$ years is $1 - (1 - \text{rate}/100)^t$.

Your tasks.

1. Finish expected. It gives the pregnancies expected in a year among couples couples.
2. Finish chance. It gives the chance of at least one pregnancy in years years, as a number from $0$ to $1$.
3. Finish best. It gives the name of the method with the lowest rate.