The Corner

Bleeeaaaagh!

Don’t you hate math?

“Hi–You write: ‘Note that the final, jumbled-up three digits can’t divide EXACTLY by 9 (giving remainder zero), because then so would my “N,” which would mean you hadn’t removed a digit….’ Actually, If your removed digit is a nine, then the ‘final, jumbled-up three digits’ will still be divisible by nine.

“But the method still gives the right result. (9-0 = 9, the removed digit.)

“That’s why they forbid you to remove a zero; without that rule, the method can’t distinguish whether you remove a nine or a zero.”

[Derb] The reader is quite correct. By spotting my error, however, he has FORFEITED HIS CHANCE TO WIN THE MUCH-COVETED PRIZE.

John Derbyshire — Mr. Derbyshire is a former contributing editor of National Review.

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