A basic concept of probability that appears frequently on the SAT Exam is that of the mean or average of a set of data.
•To determine the average, also known as an arithmetic mean, of a set of n numbers, simply add all the numbers and divide by n.
–Example: Find the mean of 34, 21, and 15.
Find the sum. 34 + 11 + 15 = 60
Divide by n = 3. 60/3 = 20.
•If the average of a set of numbers is known and all but one number is given, determine the missing number using the formula below.
–Sum of n numbers = Average x n
–Example: If the average of 3 numbers is 10 and two of the numbers is 10 and 5, find the third number.
Average x n = (10) (3) = 30
Sum of n numbers = 10 + 5 + x = 15 + x
Missing number: 30 = 15 + x à x = 15