A Walk in the Plane

Given a Cartesian coordinate system x-y in the plane. You need to get from the point (1,0) to the point (2006,2005), where in each step you are allowed to move one unit up (in the positive direction of y) or one unit to the right (in the positive direction of the x-axis).

a. In how many different paths can the task be performed?

b. In how many different paths can the task be performed if it is forbidden at any stage to pass through a point on the line x=y?

Difficulty level (1 very easy - 10 very hard): 7

Topics:
Combinatorics -> Combinatorial Geometry Combinatorics -> Binomial Coefficients and Pascal's Triangle Geometry -> Plane Geometry -> Plane Transformations -> Congruence Transformations (Isometries) -> Reflection
Sources:
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