Mathex 2012

On Friday the 10 August 2012, the manawatu mathex took place with lots schools taking part in the huge event. There were catagorys for year 5, 6, 7 and 8 each one with harder questions then the last.
Our teams didn't come first but gave it they're best effort possible! Could you answer this question?
100 green bottles are standing on a wall.  If a sharp shooter can knock off 3 green bottles every minute, how many minutes until there are 85 bottles left standing? Hard? well teams had to answer 20 questions similar or completly different to that.