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

Calculate the rent increase:  It's $900-$800 = $100.

Now divide this increase by the original rent:

$100
= 1/8, or 0.125.
$800

Mult. this by 100%, we get 12.5% (answer)