Bunuel
How many ordered pairs of positive integers (x,y) satisfy both of the inequalities \(2x+y<10\) and \(x-y>-2\)?
We have the inequalities y < -2x + 10 and y < x + 2. I'd draw a picture here; the inequality y < -2x + 10 just describes all the points below the line y = -2x + 10, and the inequality y < x + 2 is all the points below the line y = x + 2. If you draw both lines, remembering x and y are positive, we want all the points with integer coordinates contained in a quadrilateral with vertices at (0, 0), (0, 2), (5, 0) and another point in the first quadrant we don't need to solve for. From the picture, you can see which of the two lines produces the relevant boundary for different values of x.
When x = 1, because y < x + 2, substituting for x we know y < 3, so we have two points, (1, 1) and (1, 2)
When x = 2, again because y < x + 2, we know y < 4, and we have three points
When x = 3, it's now the other line that matters: y < -2x + 10, so y < 4, and we have three points
When x = 4, y < -2x + 10, and y < 2, so we have only the one point (4, 1)
When x = 5 or x > 5, we have no points, because the inequality y < -2x + 10 tells us y is negative
Counting our points, we have nine of them.