The concentration of a solution

The program holds four salt solutions, each as grams of salt and grams of water. You will make it find the mass by mass percentage of each, the salt needed for $250\ \text{g}$ of a $6\ \%$ solution, and how much salt stays undissolved when $30\ \text{g}$ is stirred into $70\ \text{g}$ of water at $20\ ^\circ\text{C}$. Then it draws a bar chart of the percentages.

The idea. Mass by mass $\% = \dfrac{\text{solute}}{\text{solute} + \text{solvent}} \times 100$. At $20\ ^\circ\text{C}$, $36\ \text{g}$ of salt dissolves in each $100\ \text{g}$ of water.

Your tasks.

1. Inside the loop, work out pc, the percentage of salt in each solution.
2. Find salt_needed, the grams of salt in $250\ \text{g}$ of a $6\ \%$ solution.
3. Find extra, the grams of salt that cannot dissolve when $30\ \text{g}$ is added to $70\ \text{g}$ of water.