180 Days of Ideas for Discussion in Math Class.
(as of 9July2014, we're in overtime!)
4 inches by 28000 square miles is (4)(28000)(63360)^2 cubic inches, which corresponds to about 1.946 trillion gallons (using 231 cubic inches for one U.S. fluid gallon), so this part seems O-K. But 486 goes into 1900 about 4 times, which means the FULL BATHTUBS each have 4 gallons in them . . .
Oh, you silly mathematicians are always going on about not assuming anything (it makes an ass out of you and me, although I don't see why I'm an ass for something you did). Maybe his bathtubs only hold four gallons. Maybe they're even four dimensional! It's not his fault that he's not limited to your "real world" bathtubs.