Firstly, you decide about the condition of getting good, average and poor level.

Here, in IF() function,  if the average score of the student is higher than 70, the level will be "good".

If the average score of the student is higher than 20, the level will be "average"; if not, the level will be "poor".

Since there are three different conditions, there should be two IF() functions, namely nested IF() functions.