At a pet store, there are 30 aquariums.
Of the 30, 20 have fresh water with
15 fish each. The other 10 aquariums
have salt water with 6 fish each. Which
equation could be used to find f, the
total number of fish at the pet store?
You are just find how many fish are salt water and how many freshwater and adding them together. The total number of aquariums is irrelevant in this case.
