Do you think he might have skipped from Venus to earth? Still a mistake, but that’s the only way he gets 50. Then the only error is 17-8 = 7, which is odd, but more understandable.

Laurel Pollard

I wonder if the student slipped to the Earth diameter when s/he went back to check the work. At a glance Earth & Venus read the same except the 50 stands out.

Michael Paul Goldenberg

First, I’m never happy when the student has proper units available and doesn’t use them. I’m not sure if math teachers in elementary or middle school grades (or just “some math teachers” in random grades) don’t think units are important, or if they’re simply so thrilled if the student gets the numbers right that they don’t want to say anything to reduce confidence and satisfaction, but without the units, these answers are all meaningless.

That said, I’d assume that the student got 700 from subtracting 100 from 800, then perhaps subtracted 4000 from 12000, got 8000, but somehow noticed that s/he probably needed to “borrow” and so dropped the thousands to 7000. Other possibilities included actually not knowing what 12000 – 4000 equals. As for changing the answer to 7750, is that adding onto or subtracting from the previous answer? It’s hard to know, exactly. The above suggestions about misreading which planet(s) were involved on second viewing is certainly feasible.