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4. Suppose southerly routes through Pascal's Triangle are forbidden to use the location (4,2), which is where there is normally a 6. Call paths that avoid this location allowable paths. (a). How many such allowable paths are there from the top down to the location (7,3), where the first 35 is normally written? (b). Give an expression for the number of allowable paths from the top (0,0) to any location (x, y).