You can put this solution on YOUR website! To find an average simply add up all the numbers in the group and divide the result by the amount of numbers in that group.
Here you have a group of 2 numbers: 54 and 81.
To find their average you need to add them up and then divide the result by 2 (because that is amount of numbers in that group).
So... 54 + 81 = 135
135/2 = 67.5
67.5% is the average