Attached is a visual that should help. Notice that multiplying by 6/5 is an easier way to get to "1/5 greater than." In other words, (6/5)x = "1/5 greater than x"
EDIT, 2022: Note that you can also simply try the answers using the backsolving method. Start with Choice C and adjust accordingly.
C) 4 -> 6 -> 7. Going from 6 to 7 is an increase of 1/6, not 1/5. Hence, 1/2 feet per year is too small. Move down to a larger answer.
D) 4 -> 20/3 -> 24/3. diff / orig = (4/3) / (20/3) = (4/3) x (3/20) = 4/20 = 1/5. Thus, going from 20/3 to 24/3 is an increase of exactly 1/5 — as is also true of going from 20 to 24. (Since the denominator is the same, you can ignore it.) Correct!
E) 4-> 44/5 -> 56/5. diff / orig = (11/5) / (44/5) = 1/4. Too big.
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