Ask Mar­i­lyn

Chattanooga Times Free Press - Parade - - Healthy Stay - By Mar­i­lyn vos Sa­vant Send ques­tions to mar­i­lyn@ pa­rade.com

My high school math teacher in­tro­duced us to a num­ber with a unique prop­erty: You could mul­ti­ply it by any num­ber, and the re­sult was es­pe­cially in­ter­est­ing. But I don’t re­call any­thing more than that. Do you know the num­ber?

—Gary Freed, Mesa, Ariz. You prob­a­bly mean the num­ber nine. Mul­ti­ply nine by any whole num­ber (ex­cept zero, of course), then re­peat­edly add the dig­its of the an­swer un­til you get a sin­gle digit, and that digit will al­ways be the num­ber nine! Ex­am­ples:

1,357 x 9 = 12,213

(1 + 2 + 2 + 1 + 3 = 9)

8,642 x 9 = 77,778

(7 + 7 + 7 + 7 + 8 = 36; 3 + 6 = 9)

Try it your­self and see, read­ers.

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