If that is the way you're explaining it to her, then I would say: stop.

perl -wle 'print ((5/3 + 7/13) / (5/2 - 3/7));' ==> prints 1.06454465075155
perl -wle 'print ((39 + 3 /13) / (35 - 9/21));' ==> prints 1.1347743165925

Are you sure you know math? Or is it Perl that doesn't know math? :)

Now back on topic, are you looking for a fully-featured calculator? or just a basic one (one that can only handle +, -, *, /)?