How many ways are there?

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How many ways are there to put 14 identical objects in 3 distinct boxes with at least 8 objects in one box?

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You have to find a unique way to describe a possible placement of the objects, that makes counting easy. Here is a solution.

  • Choose the box that has at least $8$ objects: $3$ choices.
  • For each of the preceding choices, let $k$ be the number of objects in this box ($8\le k\le 14$).
  • For each $k$, choose the number of objects in the leftmost remaining box ($0$ to $14-k$): $15-k$ choices.
  • The last box gets the remaining.

This uniquely describes the three boxes.

How many choices now?

$$3\sum_{k=8}^{14} (15-k)$$