Brad's average score after 6 rounds was 81. for the first 5 rounds his scores were 78,86,82,81 and 82.what was his score on the 6 round?

There is a "formula" to solve this, but let's use some logic, instead. If his average was 81 after 6 rounds, he must have had 6*81 = 486 total strokes.

So, totaling his first 5 rounds gives.......409 strokes. So, his last round must have been 486- 409 = 77.