Multiples — Definition & Examples

Cats age differently from humans so we say a cat has a certain age in "cat years" versus human years. Relative to humans, cats age quickly for the first two years and then slow down. A cat that is one year old in human years is $15$ years old in cat years, and a cat that is two years old in human years is $25$ years old in cat years. After the second year, we say four cat years are added for every human year that passes. If a cat is $9$ in human years, how old are they in cat years?