The decay law in code

You will write three small functions about radioactive decay.

The idea. The decay constant is $\lambda = \dfrac{\ln 2}{T}$ and the mean life is $\tau = \dfrac{1}{\lambda}$. The fraction of the nuclei that are left after a time $t$ is $2^{-t/T}$. Use the same unit of time for $T$ and $t$.

Your tasks.

1. In decay_constant, return $\lambda$ for a half-life half_life.
2. In mean_life, return the mean life.
3. In fraction_left, return the fraction of the nuclei left after a time t.