Consider the following public goods game. There are more than 2 players. Each player is given $20 at the start. Players individually decide how much of the $20 to invest in the public good. Each $1 invested in the public good generates a benefit of $2. A player's payoff is the $20 minus the amount they invested plus an equal share of the total benefit.

a) What is the Nash equilibrium?

b) If players could decide on investment collectively, how much would be invested?