Number Properties Puzzle – Challenge 1

The correct answer is C.

To answer this question, you can add all the multiples of ten from 110 to 990 and then divide the answer by the number of numbers. As you can imagine, this method takes a lot of time. Another way to solve this problem is by finding the average of the first number 110 and last number 990 (110 + 990 = 1100 ÷ 2 = 550). Do the same thing for the next two numbers 120 and 980. Continue the process for other pair of numbers; 130-970, 140-960, etc. The average of all pairs is 550. Therefore, the average (arithmetic mean) of all the multiples of ten from 110 to 990 inclusive is 550.