Can you really have divisions and additions within the .htacces inside a ReWrite Rule?
RewriteRule ^index.php?page=([0-9]+$) newPage/***($1 div 2)+5*** [NC,R=301,L]
will translate :
No, I do not think you could do.
If you're experienced in spend scripting, you may have the ability to make use of a RewriteMap. It may make use of an exterior program because the rewrite handler it's however difficult to find good good examples with this. I have not used at all it myself.
If you're proficient mainly in PHP, this can be a project for within the PHP script.