Rates
When we talk about rates, we usually need to perform a division, or a ratio between two quantities.
The time is often used as the divisor because we want to know how fast or slow things are happening.
In the case of Jordan, we know he paints 18 square feet every 15 minutes.
The constant rate, or ratio is the division between his painting area by the time taken, that is:
constant rate = 18 square feet / 15 minutes
constant rate = 1.2 square feet / minute
It means Jordan paints 1.2 square feet by minute