I cannot calculate 16 units of x, only 24 units of x by calculating the expression of the PPF. Why is A the right answer?

The following TWO questions refer to the PPF diagram below. 10 PPF X 40 4. What is the MARGINAL cost of producing good y? a) 1/4 of a unit of x. b) 1/4 of a unit of y. c) 4 units of x. d) 4 units of y. 5. What is the cost of producing FOUR units of good y? & a) 16 units of X. b) 4 units of x. 9 = kxto c) 1/4 of a unit of x. 401+10 =0 d) 40 units of x. X 401 =-10 k= -4