To find the average of two numbers, I need to add them together and divide by two. So I'll add 1.5 and 4.5, which gives me 6. Then I'll divide 6 by 2, which gives me 3. Therefore, the average length of the two pieces of string is 3 inches. 
One piece of string is 1.5 inches long and another piece of string is 4.5 inches long. What is the average length, in inches, of these two pieces of string?