from the information you can deduce that the first day you receive 6 calls,
the second day you receive twice calls..
the third day you receive twice calls that the latest day, then...
and so on... this is a progretion and we need find what day that progretion becomes 300 calls.. we can put that progretion in a table to understand better..
Finally, the seventh day, the police received more than 300 calls!