How to Calculate the Alcohol Content of Your Cocktails Question

I’m working on a physics question and need an explanation and answer to help me learn.

A bartender pours 100 ml of tonic water into one glass and 100 ml of gin into another. He then takes a 30 ml shot glass and scoops a shot of gin into the tonic and gives it a good mix. He then takes the shot glass and transfers 30 ml of the mix back into the gin glass. Is there more tonic in the gin glass, or more gin in the tonic glass?