This diagram represents a street layout Tho horizontal distance between each street is 100 feet. The vertical distance between the yellow squares is also 100 feet, while the vertical distance between the red triangles is 50 feet. These proportions are the same for all intersections.
The shortest distance between the circles marked A and B is 800 feel. How many unique ways are there to travel from A to B In 800 feet?
Given that every step you make has to be either towards the east or the south, you are only allowed to walk along the colored lines.
Specifically, you'll start walking along the green line. At some point you'll have to walk along a blue one. And then your only choice is to keep walking along the red one until you reach point B.
So every blue line corresponds to a possible route choice. In the two extreme choices, you'll walk only along two different colors instead of three.