The average would be each number of cones sold per day added, and divided by the number of days (3). So this can be translated into the equation (67 + 59 + x)/3 = 60/1. Simplified, that is 67 + 59 + x = 180, then 126 + x = 180, then x = 54.
You can check by adding 67 + 59 + 54, which is 180, and divide by 3 days which is 60. So 54 cones were sold on Sunday.
