As we can see in the graph, for 10 tickets sold ( x-axis) we donate $5 (y-axis) the point is (10,5)
So; a = 5
To donate $10 (y-axis) we have to sell 20 tickets ( x-axis)
The point is ( 20,10)
So, b = 20
For 30 tickets sold (x-axis) the money donated is 15 (y-axis)
The point is : ( 30,15)
c= 15
For 40 tickets sold (x-axis) the money donated is 20 ( y-axis)
Point: (40,20)
d=20