Book Code Answers

From any ISBN code we can calculate a 'check
total' by multiplying the digits in the code by

Try this for yourself with any ISBN number and the check **total
will always be divisible by 11**. This **calculation is used to check that
the book number has been copied down accurately and thus to avoid errors in
placing orders.**

What I noticed is that all the answers have one thing in common. **They
are all divisible by 11**.

In the ISDN code **0−947?95−24−3**

the **question mark must be replaced by the number 7**. This is the
smallest number that gives a check total that is exactly divisible by 11. See
the working below.

Multiplier | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | Total: |

ISBN Number | 0 | 9 | 4 | 7 | 7 |
9 | 5 | 2 | 4 | 3 | |

Product | 0 | 18 | 12 | 28 | 35 | 54 | 35 | 16 | 36 | 30 | 264 = 24 x 11 |