I recently read __this question__ regarding information gain and entropy. I think I have a semi-decent grasp on the main idea, but I'm curious, what to do with situations such as follows:

If we have a bag of 7 coins, 1 of which is heavier than the others, and 1 of which is lighter than the others, and we know the heavier coin + the lighter coin is the same as 2 normal coins, what is the information gain associated with picking and weighing two random coins them against each other?

Our goal here is to identify the two odd coins. I've been thinking the problem over for a while, and can't frame it correctly in a decision tree, or any other way for that matter. Any help?

EDIT: I understand the formula for entropy and the formula for information gain. What I don't understand is how to frame this problem in a decision tree format.

EDIT 2: Here is where I'm at so far:

Assuming we pick two coins and they both end up weighing the same, we can assume our new chances of picking H+L to come out to 1/5 * 1/4 = 1/20, easy enough.

Assuming we pick two coins and the left side is heavier. There are three different cases where this can occur:

HM: Which gives us 1/2 chance of picking H and a 1/4 chance of picking L: 1/8 HL: 1/2 chance of picking high, 1/1 chance of picking low: 1/1 ML: 1/2 chance of picking low, 1/4 chance of picking high: 1/8

However, the odds of us picking HM are 1/7 * 5/6 which is 5/42

The odds of us picking HL are 1/7 * 1/6 which is 1/42

And the odds of us picking ML are 1/7 * 5/6 which is 5/42

If we weigh the overall probabilities with these odds, we are given:

(1/8) * (5/42) + (1/1) * (1/42) + (1/8) * (5/42) = 3/56.

The same holds true for option B.

option A = 3/56

option B = 3/56

option C = 1/20

However, option C should be weighted heavier because there is a 5/7 * 4/6 chance to pick two mediums. So I'm assuming from here I weight THOSE odds.

I am pretty sure I've messed up somewhere along the way, but I think I'm on the right path!

EDIT 3: More stuff.

Assuming the scale is unbalanced, the odds are (10/11) that only one of the coins is the H or L coin, and (1/11) that both coins are H/L

Therefore we can conclude:

(10 / 11) * (1/2 * 1/5) and

(1 / 11) * (1/2)

EDIT 4: Going to go ahead and say that it is a total of 4/42 increase.