Assembly starts at three o'clock and ends at quarter past three how many minutes does assembly last

 "15 minutes" .

Start time:  "3:00" .

End time:  "a quarter past three" = (1/4) hr after "3:00" ;
                                                        =  (1/4) * 60 minutes (after "3:00"); 
                                                        =  15 minutes (after "3:00").
End time:   "3:15" 

The answer is:  "15 minutes" .

The answer might be a