A student has made a 79, 56, 91 and 72 on four tests. Determine the possible scores on the fifth test that will result in a 75 or higher average

The score on the fifth test we will call 'x'

so the average after the fifth test will be:

(79 + 56 + 91 + 72 + x)/5

or

(298 +x)/5

or

59.6 + x/5

so what we want is:

75 <= 59.6 + x/5

15.4 <= x/5

77 <= x

In order to get at least a 75, the student needs to score a 77 or higher.