1089

Choose a 3 digit number whose first and last digits differ by at least 2.

E.g 853

Reverse this number

I.e. 358

Subtract the smaller from the larger

I.e. 853 – 358 = 495

Add this number to the reverse of this number

I.e. 495 + 594 = 1089

Why? (That’s the fascinating part of maths.)

Thanks to Alex Bellos’s book “Alex’s Adventures in Numberland” for rekindling my interest in numbers.

My demonstration of why 1089 is the answer:

hundreds tens units notes
start a b c a>c+1
invert c b a
subtract a-(c+1) 10+b-(b+1) 10+c-a +10 carry 1 in tens,units
simplify 9
invert 10+c-a 9 a-(c+1)
add 10 8 9 Carry 1 in tens
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4 thoughts on “1089

  1. i love stuff like this; my eighth grade math teacher showed me the magic of nine, and I’ve been playing ever since. he was a dreadful teacher who loved math but just couldn’t teach it. Poor soul.

    Like

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