1. A robot starts at (0,0). It can move either (+1,0) or (0,+1), i.e. up or right. For example, it can move to the point (1,2) by applying (+1,0) then move (0,+1) twice.
(a) How many ways can the robot get to the point (10,10). (Hint: Start figuring out how many ways the robot can move to (1,1), (2,2),... first).
(b) Someone put a boulder at (5,8) that the robot cannot pass through. Now how many paths are there to (10,10)?
(c) Now in addition to the boulder at (5,8), there is another boulder at (4,4). How many paths to (10,10) are there now? Hint: INCLUDE some, EXCLUDE others...