Question 9886
If 89 is the average then the total of 5 tests must be 89x5 = 445

Given the first four test scores 82, 93, 77, and 88 whose sum is =340

So number of marks required to get an average of 89 = 445-340  = 105

Another way of doing this problem is as follows

Since 89 is the average, add the difference between the average and the remaining numbers

So 82 - 89 = -7
93- 89 = +4
77-89 = -12
88-89 = -1

-7 + +4 + -12 + -1 = -16

Which means the final score is 16 more than 89 

89+ 16 = 105