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