A student scored 84, 78 and 72 in three tests. The least score needed in a fourth test so that the average of all four tests is at least 80 is:

If the fourth score is s, the condition is (84 + 78 + 72 + s)/4 ≥ 80, i.e. 234 + s ≥ 320, so s ≥ 86. With 86 the total is 320 and the average is exactly 80; with 84 the average is only 79.5.