[ctv1116]

This is basic, its called pot equity. You are going to win this pot 80% of the time (the exact % is not all that important, you just know its significanly greater than 50%). You can divide the pot into the $200 already in the pot, and the bet you are deciding to throw in. What's your expected value on the $40 (20 from you and 20 from your opponent) that goes in on the turn? You get 80% of it and your opponent gets 20% of it. So the turn bet gets you an expected value of $12.