Three corners of a rectangle drawn on a coordinate grid are (1, 1), (6, 1) and (6, 4), and its sides run along the grid lines. What is the fourth corner?

Erklärung

The missing corner sits directly above the corner at (1, 1) and directly left of the corner at (6, 4), so it borrows the 1 across from one of them and the 4 up from the other. Reversing those two numbers lands part way along the bottom side instead of at the corner that is missing.