A student has scores of 85,83,98 and 77 on four quizzes. What must she score on the fifth quiz to have an average of at least 84?

We need to find the solution to:
(85 + 83 + 98 + 77 + x) / 5 >= 84 where x is the score in the last test
This is the same as:
(343 + x) / 5 >= 84
Multiplying both sides by 5 we get
343 + x >= 420
Subtracting 343 from both sides we get
x >= 77

There for the student must get a score of at least 77 on the last tests.