Payoff Matrix
The size of the circles represents the probability of the payoff occurring, given the other player's strategy.
There are two pure-strategy Nash Equilibria: both go to the opera, both go to the fight.
There's also a mixed strategy Nash equilibrium. In order to achieve it, both players need to be indifferent between going to the opera and going to the fight, and therefore willing to play a mixed strategy. Can you figure out what that equilibrium is?
The size of the circles represents the probability of the payoff occurring, given the other player's strategy.