You have a list of five positive integers; 10 is the smallest, 15 is the mode (or the number that repeated most), 30 is the largest. Your average is 18, which means (10 + 30 + 15 + 15 + a)/5 = 18 Since 15 is repeated most, that means that there is more than one of this number. You already have 10 and 30, and you have an "a" value, so that means 15 repeats twice in the list. To find the average of something, you add up all values and divide the sum by the total number of numbers. Since we are finding the sum of 5 numbers, we divide the sum by 5. Set this equation equal to 18 (a given detail) and solve. (10 + 30 + 15 + 15 + a)/5 = 18 Multiply both sides by 5 first 10 + 30 + 15 + 15 + a = 90 70 + a = 90 a = 20 Thus, your value is 20.
