Four tests give an average of 15 marks. A fifth test gives 40 marks. What is the new average?

1. The first four tests total 15 x 4 = 60 marks. 2. Adding the fifth test gives 60 + 40 = 100 marks over 5 tests. 3. New mean = 100 / 5 = 20. 18 and 22 come from guessing near 20, and 25 is the plain average of 15 and 40 ignoring how many tests there were. _Source: Wikipedia, Arithmetic mean, Definition_