Answer:
Market price , quantity = 50 , 50 
Social price , quantity = 65 , 35
Deadweight Loss = 112.5 
Step-by-step explanation:
In perfectly competitive markets , optimal equilibrium is where :
[Marginal Revenue = Price] = [Marginal Cost]
[ MR = 100 - Q] = [MC = Q] 
100 - Q = Q 
Q + Q = 100 
2Q = 100 → Q = 100/2
Market Q = 50 
P = 100 - Q → P = 100 - 50 
Market P = 50 
Socially efficient markets are where : additional (external) cost or additional benefit is also used to determine socially optimum quantity 
[ MR = 100 - Q] = [TC = MC + EC]
100 - Q = Q + 30 
100 - 30 = Q + Q 
2Q = 70 → Q = 70 / 2
Social Q = 35
P = 100 - Q → P = 100 - 35 
Social P = 65 
Deadweight Loss 
1/2 [ (P2 - P1) (Q1 - Q2)] 
1/2 [ (50 - 35) (65 - 50) ] 
1/2 (15x15)] → 1/2 (225) 
Deadweight Loss = 112.5