In this case, every bottle has 500 mg of caffeine with a standard deviation of 25 mg. The equation of a bottle will be: 500 +/- 25mg caffeine per bottle.
To calculate a carton containing 30 bottles, you just need the equation with 30. It will become: (500x30) +/- (25x30)mg= 15000 +/- 750 mg of caffeine per carton