-- 20.9 is roughly 30.
-- 2.9 is roughly 3.
-- If Sue started with 30 and subtracted 3 from it, she would have 27.
-- 1.8 is nowhere near 27. It's not a reasonable answer.
(Actually, 20.9 - 2.9 = 18 . So you can see where maybe she got her
digits from, but she made some kind of bad mistake in handling them.)
