16. From the permutation $a_{1} a_{2} \cdots a_{n}(n \geqslant 10)$, select 3 elements $a_{i}, a_{j}$, and $a_{k}(1 \leqslant i<j<k \leqslant$ $n)$, such that in this permutation, there are at least 3 elements between $a_{i}$ and $a_{j}$, and at least 4 elements between $a_{j}$ and $a_{k}$. How many different selection methods are there?
16. $\binom{n-7}{3}$.