In order to find the average of a certain amount of numbers, you add those numbers together and divide by how many numbers there are. In this case, you would reverse that method to find the last number. We know that there were four readings, and we have three of them and the average. Multiply the average (84.75) by the number of readings (4), and you get the sum of all the readings (339). Then you subtract each of the readings that you already know to isolate the last reading. 339 - 112 (first reading) = 227, 227 - 68 (second reading) = 159, and 159 - 85 (fourth reading) = 74. Therefore, the third reading must be 74.
