MathNet 2021 — Part 04 Question 49

Find the number of 10-digit numbers $\overline{a_{1} a_{2} \cdots a_{10}}$ which are multiples of 11 such that the digits are non-increasing from left to right, i.e. $a_{i} \geq a_{i+1}$ for each $1 \leq i \leq 9$.