The expression is r = 500 • 2, where “r” represents the rate.
This is pretty much a "common sense" question. It'd be best to get rid of the dirty sanitizing water and start fresh; in other words, he should change (throw out) the dirty water and make a fresh sanitizer.
By the way, this is the Math section; next time, please choose a different section more suited to your questions.
* r^2