I think it's misleading to introduce this "third" quantity of 0.5x Yes, it is misleading, that's my point :) Put it another way: you have two quantities in tw

I think it's misleading to introduce this "third" quantity of 0.5x Yes, it is misleading, that's my point :) Put it another way: you have two quantities in two envelopes, A and B. If you choose B, what is the probability that the other envelope contains a 3rd quantity, C? The probably of that is zero, because C was not present in the initial two envelopes. Still another way of putting it: I will place $10 in an envelope, and $20 in another envelope, then ask you to choose one. What are the chances that the other envelope contains $40? The chance of that is zero, because the only two amounts present at the start of the trick were $10 and $20.