Welcome to the community 
If you use / or * Make will see this as text (string) values. So it won’t do math with them.
So you should be able to fairly easily fix your issue by using the math operators (joined with brackets).
You should end up with a formula like this:
Start off by adding the brackets to your field:
Then, add your variables and insert the math functions to do the calculations:
If you then run this, it should give you the desired result:
Hope that helps!
Thomas - Nola Digital