Solution 2

First find the supply curve for an individual seller:

WTAi = 5 + 0.1*Qi

WTAi = P

P = 5 + 0.1*Qi

Qi = (P-5)/0.1

Qi = 10*P - 50

Solving for Qi at the given price:

Qi = 10*12 - 50 = 70

The individual seller's PS can be calculated from the triangle below:

PSi = (1/2)*70*(12-5)

PSi = (1/2)*70*7

PSi = 245

Since there are 20 identical sellers, the total PS is given by:

PS = 20*PSi = 20*245

PS = 4,900