well... the number of sold glasses = y = 75.

the price of a glass of lemonade = x = 80.

linear function: y = k*x + m.

so all we know is 75 = k*80 + m.

that equation has an infinte number of equations... y = x - 5, y = 2x - 85, y = 3x - 165... i guess the right answer is the functions y = n*(x - 80) + 75, where n is an integer...

edit: don't be embarassed, it's a lousy question since there are an infinite number of possible answers...