Here is how you figure this out. First, you need to find out how many total points she had from the first five tests. The way to do that is to multiply 5 by 79.
79*5 = 395
So before the sixth test, she had 395 points. So now you add in the 85 points on her sixth test. Now she has 480 points.
To get her new average, you divide 480 by 6 because that is how many points and how many tests she has now taken.
480/6 = 80
Her new average is 80%.
An average score of 79 after 5 tests means
Total score after five tests
= (Average score)*(Number of test) = 79*5 = 395
Score in sixth test = 85
Therefor: Total score after 6 tests
= Total score of first 5 tests + score of sixth test = 395 + 85 = 480
New average score after sixth test
= (Total score of 6 tests)/6 = 480/6 = 80
New average score = 80
The average is always the total marks of the tests divided by the number of tests. So the total of the first 5 tests is the average of the first 5 test multiplied by 5 which is 79*5 = 395. With 85 score in the 6th test , the total of 6 tests of Nora is 395+85 = 480. So now the average of the 6 tests of Nora is total marks obtained by her in 6 tests divided by six = 480/6 = 80