The average mean is equal to the sum of numbers divided by the total numbers, which means that the mean of 230 with 80 numbers gives that the sum is 230*80 = 18400, if you apply the formula described in the beginning. Since 4 numbers are known and are 145,156,210, and 255, their sum is 766. The sum of the list is equal to the sum of the numbers without the sum of 4 known numbers, so it's equal to 18400 - 766 = 17634, from which we can see that the Mean = (Sum Of List)/(80 - 4)
= 17634/76
= 232.02
