Riddles of what flies when its born

The one way they are similar is that they are in the same instrument family and three ways they're different are they don't have the same amount of strings, the bass has a deeper sound, and bass is bigger.

Having 2 glasses, in one is 10 cl of tonic and in the other 10 cl of fernet. Overspill 3 cl of tonic to the glass with fernet and after thorough mixing overspill 3 cl of the mixture back to the glass with tonic. Is it now more tonic in the glass of fernet or more fernet in the glass of tonic?
(Ignore chemical composition!)

Answer

There is exactly as much tonic in the glass of fernet as there is fernet in the glass of
tonic.

12, 34, 10, 11, 12, 5, 6 ,7, 13, 14, ?, ?
Logically, what two numbers should replace the question marks?

Answer

15 and 11

1 + 2 + 3 + 4 = 10, then insert next two numbers 11, 12 and add digits 1 + 1 + 1 + 2 = 5. Insert next two numbers 6, 7 and add digits 6 + 7 = 13. Insert next two numbers 14, 15 and add digits 1 + 4 + 1 + 5 = 11.

I am owned by every man, though my length differs. Their wives uses me after getting married.
What am I?

Answer

Last Name.

What name does this make?
1
2
2
1

Answer

OTTO
1 One
2 Two
2 Two
1 One

There are two identical doors, but only one has a treasure inside. Each is guarded by a robot, "X" & "Y". The doors and robots are identical in every way, except one robot always lies and one always tells the truth. But you don't know which one is the liar.
What one question, could you ask only one robot, and know which door to open to find the treasure?

Answer

You ask the first robot, "What would the other robot say if I asked if there is treasure behind his door?" (The truth is the opposite of his answer.) If the robot "X" says 'yes' then there is no treasure behind robot "Y"'s door. If robot "X" says 'no' then the treasure his behind robot "Y"'s door. Hint: It has to do with the math principle 1x(-1)=-1 and (-1)x1=-1. If you involve both robots you are guarenteed a lie to your question.