So that's the equation I came up with. Maybe I'm over complicating the question but it seems correct. Its a 4th grade math book for crying out loud, so could it really be a "Quadratic equation" :) Some 4th grade math books have questions like, "Pete and Repeat were on a boat...Pete fell off....who was left?" :)
Or my personal favorite..."why is a rooster on top of a weather vane?"