How many ordered pairs $(x,y)$ of positive integers satisfy the inequality $4x+5y < 20$?
See the graph here : https://www.desmos.com/calculator/jn9lm00kci
The pairs are
(1, 1) (2, 1)
(1, 2) (2, 2)
(1, 3) (3, 1)
+40 
