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Oct 15 · 4 min read

Ethan had covered a long journey trying to solve problem ABCDE * 4 = EDCBA. Finally only the last unknown remained in front of us. Even better, we had narrowed down the range of possible values of C to just two digits: 8 or 9.

ABCDE * 4 = EDCBAA = 2E = 8B = 1D = 7C = 8 or 921C78*   4-----87C12

Word “Congrats” was in my mouth but the moment refused to come, because after quite a few minutes of mental calculation (I guessed) Ethan told me that neither 8 nor 9 worked. Ethan loves math and this small piece of multiplication should have been a piece of cake to him. “Try again and do it carefully. We don’t want to lose an answer if there is actually one”, I told him. Unfortunately he might get stuck somewhere and couldn’t break through.

Better to pull him out of the trap with something different. “OK, grab a piece of paper and redo the 21878 * 4 and 21978 * 4”, I said.

“Oh, I made a mistake in one of the carryings. Now I get it. 8 is bad. The answer is 9 for letter C.” Ethan spotted where he was stumbled this time.

“We wasted the big 5 minutes in the tiny arithmetic. Could you spank yourself 5 times and let me hear it?” I said.

“Bang, bang, bang, bang, bang” came from the other end.

After the class I posted a message in our small WeChat group to share the story with his mother: “Today Ethan punished himself with 5 spanks, because he got 21978 * 4 wrong for too long.” What happened next drove his mom to tears, and me too.

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