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