Oracle Database – Dynamic SQL

I want ot be able to store a formula as a string in the database and then
apply it to calculate a price.

The problem I’m having is that Oracle does not seem to recognise the + * /
or – operators when in a char string.
Example
Char string(3 * 30) / (2*5) returns ‘ (3 * 30) / (2*5)’ and not 9

Can anyone help me solve this problem?