Samir has xx pennies and yy nickels. He has no less than 20 coins worth no more than $0.40 combined. Solve this system of inequalities graphically and determine one possible solution.
a) (xx, yy) = (10, 10)
b) (xx, yy) = (15, 5)
c) (xx, yy) = (5, 15)
d) (xx, yy) = (20, 0)