y = total cost
x = amount of toppings
z = # of patties
y = 4z + 0.75x
For one burger with 1 topping, the equation becomes y = 4 + 0.75
1 topping: $4.75
4 toppings: 4 + (0.75 x 4) = $7
6 toppings: 4 + (0.75 x 6) = $8.50
10 toppings: 4 + (0.75 x 10) = $11.50