{(2, 3), (1, 5), (2, 7)} - NOT because 2->3 and 2->7 {(11, 9), (11, 5), (9, 3)} - NOT because 11->9 and 11->5 {(3, 8), (0, 8), (3, -2)}
- NOT because 3->8 and 3->-2
X+y=15 0.3x+0.6y=0.4*15=6 solve the system of equation. Multiply equation one by 0.3: 0.3x+0.3y=4.5 subtract the new from the second equation: 0.3y=1.5 y=5 x=10