7. Arrange $\{1,2, \cdots, 7\}$ randomly into $\left\{a_{1}, a_{2}, \cdots\right.$, $\left.a_{7}\right\}$. The probability that $a_{1}+a_{2}+a_{3} \geqslant a_{5}+a_{6}+a_{7}$ is $\qquad$
7. $\frac{73}{140}$