7. (5 points) In the figure, three equally sized equilateral triangular transparent sheets of glass paper, each divided into 49 smaller equilateral triangles of the same size, have 16 of the smaller triangles shaded on each sheet. If these three sheets are overlapped, the number of shaded smaller equilateral triangles seen is $\qquad$.

【Answer】Solution: $16 \times 3-3+1-15 \times 2+15$,
$$
\begin{array}{r}
=48-2-15, \\
=31 \text { (pieces), }
\end{array}
$$

Answer: The number of small equilateral triangles seen in the shadow is 31; hence the answer is: 31.