HMMT November

Combinatorics Difficulty 5.0 AIME, harder Find the answer

Consider the paths from $(0,0)$ to $(6,3)$ that only take steps of unit length up and right. Compute the sum of the areas bounded by the path, the $x$ -axis, and the line $x=6$ over all such paths.

Your answer

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We see that the sum of the areas under the path is equal the sum of the areas above the path. Thus, the sum of the areas under the path is half the area of the rectangle times the number of paths, which is $\frac{18\binom{9}{3}}{2}=756$ .

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