Twelve integers $a_1,a_2,a_3,\dots ,a_{12}$ are arranged in order on a number line. The integers are equally spaced and have the property that
$1\le a_1\le 5$, $18\le a_2\le 20$, and $170\le a_{12}\le 175$.
What is the sum of digits of $a_{10}$?