Radius and density in code

You will write three small functions about the size of a nucleus. They use $R = R_0A^{1/3}$ with $R_0 = 1.2$ fm.

The idea. The mass of a nucleus is $A$ atomic mass units, with $1\ \text{u} = 1.660539 \times 10^{-27}$ kg. The volume is $\tfrac{4}{3}\pi R^3$, with $R$ in metre.

Your tasks.

1. In radius_fm, return the radius in femtometre.
2. In density, return the density in kg/m³. Change the radius to metre first.
3. In mass_number, return the mass number of a nucleus of radius r femtometre, rounded to a whole number.