The box-and-whisker plot is just a fancy way of dividing stuff into quarters.
1/4 (a quarter) of the tanks use 350-450 gallons.
1/4 use 450-500 gallons.
1/4 use 500-600 gallons.
1/4 use 600-750 gallons.
Two of those are above 500.
1/4 and 1/4 makes 2/4 or 1/2.
