Mix an alloy

A foundry mixes metals by a recipe in percent. You will make the program work out the mass of each metal for any amount of alloy, check that a recipe adds up to $100\ \%$, and find the copper for a bronze bell.

The idea. Mass of a metal = total mass × its percentage ÷ $100$. A recipe is only valid if its percentages add up to $100$.

Your tasks.

1. In masses, return a dictionary of the mass of each metal.
2. In valid, return True when the percentages add up to $100$.
3. Find copper_bell, the copper in $2.5\ \text{kg}$ of the bell bronze.