A radioactive substance decays such that its mass $M$, in grams, after $t$ hours can be modeled by the formula $M=C{\left(\frac{1}{2}\right)}^{rt}$, where $C$ and $r$ are constants. Initially, there are 800 grams of the substance. After 5 hours, 100 grams of the substance remain.

What is the value of $r$?