### I know what is wrong

Let me explain with an example. Say I prepare six envelopes. I put a $10 bill inside one, and set it aside. I distribute one$20 bill and four $5 bills between the rest, pick one at random, and give it to you along with the one I set aside. I can now truthfully say "They both contain money, one twice as much as the other." The probability that you have the smaller envelope is 50%. But this is broken down into a 40% chance that you have$5 and the smaller envelope, a 10% chance that you have $10 and the smaller envelope, and a 0% chance that you have$20 and the smaller envelope. Similarly, there is a 0% chance that you have $5 and the larger envelope, a 40% chance that you have$10 and the larger envelope, and a 10% chance that you have $20 and the larger envelope. If we call the amount in your envelope X, then X is clearly either$5, $10, or$20. BUT THERE IS NO VALUE OF X WHERE THE PROBABILITY THAT X IS THE SMALLER AMOUNT IS THE SAME AS THE PROBABILITY THAT X IS THE LARGER AMOUNT.
If I let you look in your envelope, and you see a $10 bill, there is only a (10%)/(10%+40%)=20% chance that it is the smaller amount. This is called a conditional probability; it found by dividing the probability of the outcome you know ($10) and the outcome you are interested in (smaller) happening together, by the total probability of the outcome you know. Similarly, the chance that it is the larger amount is (40%)/(10%+40%)=80%.