Remember, mean is average. So first you add them all up, and then divide by the total number. All the dollars added up and then divided by 6 equals 59. So, working backwards, if you multiply 6 (the number of categories) by 59 you get the total of all money earned from every category. 6 times 59 equals 354. But if you add what you can see right now, it only equals 274. So the t-shirt total must be the difference between the two... 80 dollars.
