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umm... I did it differently... Is this still correct?
After pouring the 3cl of tonic into the fernet, there would be 3/13 tonic and 10/13 fernet in the fernet glass (with 7/7 in the tonic glass).
When you pour the 3 cl back, you would get 1.5 cl of tonic and 1.5 cl of fernet (since they were mixed thoroughly).
That leaves 8.5/10 tonic and 1.5/10 in the tonic glass, and 1.5 tonic and 8.5 fernet in the fernet glass.
That comes with the same answer, but did I do it wrong and was just lucky, or what?
Neah bro. Here is what I see wrong with the math:
When you pour back 3 cl you are pouring back 3* 3/13 =0.7 tonic
+ 3* 10/13 =2.3 fernet
= 3.0 cl and not 1.5 & 1.5. You see what I mean. So, you end up with 7.7 & 7.7 not 8.5 & 8.5.