Sue subtracted 2.9 from 20.9 and got 1.8. explain why this is not reasonable

-- 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.)