Look on the back of any modern book and you will find an ISBN code. These
are used to identify every book that is now published. 
<p/>
For instance one of Tolkein's  books has an ISBN code on the back of



<br clear="all" /><table border="0" width="100%"><tr><td>
<table align="center"><tr><td nowrap="nowrap" align="center">
0<font face="symbol">-</font
>04<font face="symbol">-</font
>823208<font face="symbol">-</font
>4. </td></tr></table>
</td></tr></table>


<p/>
Take this code and calculate this sum:



<br clear="all" /><table border="0" width="100%"><tr><td>
<table align="center"><tr><td nowrap="nowrap" align="center">
(1&#x00D7;0)+(2&#x00D7;0)+(3&#x00D7;4)+(4&#x00D7;8)+(5&#x00D7;2)+(6&#x00D7;3)+(7&#x00D7;2)+(8&#x00D7;0)+(9&#x00D7;8)+(10&#x00D7;4) </td></tr></table>
</td></tr></table>


<p/>
Now find a small stack of books and repeat this sum for each of their ISBN
codes.
Sometimes the last number of an ISBN code is replaced with an X.
If it is count this as 10.
<p/>
Can you see what the answers always have in common?
<p/>
In the ISBN code below there is one number that has been replaced with
a ?
<p/>
Do you know what this number is for it to be a valid ISBN code?
<p/>



<br clear="all" /><table border="0" width="100%"><tr><td>
<table align="center"><tr><td nowrap="nowrap" align="center">
0<font face="symbol">-</font
>947?95<font face="symbol">-</font
>24<font face="symbol">-</font
>3</td></tr></table>
</td></tr></table>


<p/>
What do you think is the purpose of this property of ISBN codes.
<p/>