SAT Question c256b723: Percentages answer and explanation with Desmos solution

It's given that the amount of Hanna's food order was $$50$ and that Hanna gave a tip of $20\%$ of the amount of the bill. $20\%$ of $50$ can be calculated as $\left(\frac{20}{100}\right)\left(50\right)$, which yields $\frac{1000}{100}$, or $10$. Therefore, the amount, in dollars, of the tip Hanna gave is $10$.