Ernest has pretty much laid out the answer for you. You're asking for the actual syntax without showing that you have at least tried. You'll find that if you show some effort, people around here will be more willing to assist you.
Now Ernest said that in the main method (which will be in your RatioTester class) you need to create an instance of the Ratio class and then you will be able to call your method.
So you need to know three basic things here:
1) how do you create an object from a class (hint you need a
java key
word)
2) how to assign a reference to that object, and
3) how to call a method on the object.
These are all basic things that can be found in any beginner's Java text, or alternatively you can search for on the Web or on this site. There is plenty of material out there.
Good luck!
Regards,
JD