7. Let $M=\{1,2, \cdots, 17\}$. If there are four distinct numbers $a, b, c, d \in M$, such that $a+b \equiv c+d(\bmod 17)$, then $\{a, b\}$ and $\{c, d\}$ are called a “balanced pair” of set $M$. The number of balanced pairs in set $M$ is $\qquad$
7.476 .

As shown in Figure 2, the circumference is divided into 17 equal parts, and the points are labeled in a clockwise direction as $k+l(\bmod 17)$ if and only if the chord $A_{m} A_{n} / / A_{k} A_{l}$.

Notice that, for any pair of points on the 17 equally divided points on the circumference, the line connecting them is never a diameter. Therefore, there are 17 directions for all the chords (each parallel to the tangent line passing through point $A_{i}(i=1,2, \cdots, 17)$). There are 8 chords parallel to the tangent line passing through point $A_{i}$, forming $\mathrm{C}_{8}^{2}=28$ pairs of "parallel chords". Considering all 17 directions, we get $28 \times 17=476$ pairs of parallel chords, meaning there are 476 balanced pairs in $M$.