A person is to count $4500$ currency notes. Let $a_n$ denote the number of notes he counts in the $n^{th}$ minute. If $a_1=a_2=\dots =a_{10}=150$  and $a_{10}$, $a_{11},\dots$ are in an $A.P.$ with common difference $-2$, then the time taken by him to count all notes is