Solution 1

Given:

W2P = 140-2*Q
P = 40

Start by finding the quantity she will buy:

W2P = P
140-2*Q = 40
100 = 2*Q
Q = 50

Drawing the CS diagram:

Calculating the area of the triangle:

CS = (1/2)*50*(140-40)
CS = (1/2)*50*100
CS = 2,500