# of ways to go to the opposite corner. Suppose m+1 parallel (laid N-S)roads cross n+1 parallel (laid E-W) roads. In how many different ways one can move from North- East corner (of crossing) to South- West corner heading either West or South? What the simplest / attractive way (explanation) to make students of age 17yr understand while teaching in class. I observed, students really do not get the logic.