The inverse/opposite of "50% chance head, 50% chance tails" is "50% chance tails, 50% chance head". Which is the same thing. If you are looking at A alone, A i

The inverse/opposite of "50% chance head, 50% chance tails" is "50% chance tails, 50% chance head". Which is the same thing. If you are looking at A alone, A is random. If you are looking at B alone, B is random. If you are looking at A and B together, they are entangled, i.e. their probabilities are "50% chance A head B tails, 50% chance A tails B head". Random means something you can't predict (other than statistically), until it actually happens. If you know the state of the entire universe except for the particle B, you can't predict A. If you know the state of the entire universe except for the particle A, you can't predict B.