Question 169
Let $l$ be a straight line lying on the $xy$-plane. For given $20 \times 15$ points $(m, n) : (m = 1, 2, \dots, 20, n = 1, 2, \dots, 15)$ on the $xy$-plane, there are $222$ straight lines parallel to the line $l$ ($l$ itself may be considered as one of those parallel lines) and going through at least one of these points. How many straight lines are there which go through at least one of these points and are perpendicular to $l$?