This question is just a question targeted towards puzzling the solver. It is infact a very easy problem. In the given question the information's provided should be very minutely watched. First information says that there is an unknown number. The problem revolves around this unknown number and various information's show that this number is always connected with the majority information's provided.
Let us assume the unknown number = x
Then
2x -4 = x - 10
2x - x = - 10 + 4
x = - 6
So the unknown number is -6.