There are 3 coins in a bag. One is a normal coin with "heads" on one side and "tails" on the other. The second one has "heads" on both sides and the third has "tails" on both sides.

Randomly draw one coin from the bag and put it on the table without looking at the down face.

The up face is a “heads”.
What is the probability the down face is “tails”?

Please justify your answer.