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.

Of the six sides there are 3 with heads. 2 of those "heads" sides have heads on the other side. 1 of those "heads" sides has tails on the other side, So the probability of the down face being "tails" is 1 in 3.
I hope.