Since wants needs an 85 as an average and he takes five tests, he must have 85 * 5 = <<85*5=425>>425 total points.
Currently he has 85 + 79 + 92 + 84 = <<85+79+92+84=340>>340 points.
So for his fifth test he needs 425 - 340 = <<425-340=85>>85 points.
#### 85