7. (5 points) In a large box, there is a medium box, and inside the medium box, there is a small box. Place 100 marbles into the boxes, where $n$ marbles are in the large box but not in the medium box, and $m$ marbles are in the medium box but not in the small box. If the number of marbles in the small box is expressed in terms of $m$ and $n$, it should be . $\qquad$
【Analysis】Since $n$ balls are in the large box but not in the medium box, and also not in the small box, and $m$ balls are in the medium box but not in the small box, subtracting these two parts from the total gives the conclusion.

【Solution】Solution: According to the problem, the number of balls in the small box is $100-m-n$.
【Comment】Pay attention to the meaning expressed by the language, and further clarify the data to solve the problem.