The answer is 33.
Here's how I worked out the average of the integers from 25 to 41 using a Google Spreadsheet:
1. Write the numbers 25, 26, and 27 into cells:


Obviously the cells inside the brackets will depend on what cells you're using - but A2:A18 are the cells that match the example I'm using.
This will give you your answer, 33.
In reality, all you're doing is adding all the numbers up and dividing by how many numbers there are (17). This is how you find the average out.
In this case, your answer is 33.