In a perfectly competitive market, the demand curve is as follows

q_D = 150 – p for 0 ≤ p ≤ 150

The supply curve of a single firm is given by (Assume that the market consists of identical firms)

q_s^f = 15 + p for p ≥ 25

= 0 for 0 ≤ p < 25

With free entry and exit of the firms, equilibrium price will be?

25

The correct answer is Option 4: 25

In a perfectly competitive market, equilibrium is determined where market demand = market supply. 

Given the demand curve: \[ q_D = 150 - p \] - The supply curve for a single firm is: \[ q_{sf} = 15 + p, \quad \text{for } p \geq 25 \] \[ q_{sf} = 0, \quad \text{for } p < 25 \] 

Since firms are identical and there's free entry/exit, the equilibrium price will be at the minimum average cost (which in this case corresponds to the point where the firm starts supplying).

The firm's supply starts at p ≥ 25. Therefore, the minimum price for supply is 25.

In perfect competition, in long run equilibrium, price equals minimum average cost. Since this problem is asking for equilibrium price with free entry and exit, price must equal 25.

Market supply is the sum of all individual firm supplies. Since we know the price, we do not need to calculate the number of firms.