Question

In the past you paid $800 per month to rent your apartment. You now pay $900 per month for your rent. What is the percent increase in your rent?

Answer 1

If we take 800 to be the 100%, and the difference is just 100 bucks, how much is 100 off of it in percentage?

$\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 800&100\\ 100&p \end{array}\implies \cfrac{800}{100}=\cfrac{100}{p}\implies p=\cfrac{100\cdot 100}{800}$