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Transposition Fix

Age 14 to 16
Challenge Level Yellow star
  • Problem
  • Getting Started
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An ISBN Number

Suppose an operator types a US Bank check code into a machine and transposes two adjacent digits (i.e. swaps the order of two adjacent digits) for example the operator types $a_3a_2$ instead of $a_2a_3$, will the machine pick up every error of this type? Does the same apply to ISBN numbers; will a machine detect transposition errors in these numbers?

US Bank check codes are nine digit identification numbers $a_1a_2\cdots a_{9}$ using the digits 0 to 9 where
$7a_1 + 3a_2 + 9a_3 + 7a_4 + 3a_5 + 9a_6 + 7a_7 + 3a_8 \equiv a_9$ mod 10.

ISBN Numbers have ten digits $a_1\cdots a_{10}$ using the digits 0 to 9 where
$10a_1+9a_2+8a_3+\cdots +3a_8+2a_9+a_{10} \equiv 0$ mod 11.


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Prove that if a^2+b^2 is a multiple of 3 then both a and b are multiples of 3.

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a) A four digit number (in base 10) aabb is a perfect square. Discuss ways of systematically finding this number. (b) Prove that 11^{10}-1 is divisible by 100.

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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