It's tough to answer these questions without actually answering the questions (and in essence, doing your work for you.)
Think about an algebraic equation. (dependent variable) = (independent variable) + (constants), like, Y = (X + 79 + 85 + 92) / 4.
From here, it's a question of determining your variables. Do the test scores depend on the average or does the average depend on the test scores?
Substitute the given numbers for the right variables, and you'll get the answer.
In this case (and probably in most cases), the desired average depends on the test scores. So, Y = 90. X = the grade he needs to get.
90 = (X + 79 + 85 + 92) / 4
Multiply both sides by 4
360 = X + 79 + 85 + 92
360 = X + 256
104 = X
I guess Justin should have studied harder, because I doubt his professor gives extra credit to anyone, especially to such a slacker like JUSTIN. Good luck getting better than perfect on your final, buddy.
Another way to see this is the "weighted average" method, which may be relevant to other questions. Notice the part that says "each weight equally"?
That would be set up this way:
90 = (0.25)x + (0.25)79 + (0.25)85 + (0.25)92
We use 0.25 because 100 divided by 4 tests yields 25 basis points per test. In simpler terms, each test is 25% of the whole grade because 25/100 = 1/4 = 0.25
90 = (0.25)x + 19.75 + 21.25 + 23
90 = (0.25)x + 64
26 = (0.25)x
Divide both sides by (0.25) (or multiply both sides by 4)
104 = x
Either way, Justin's SOL (simply out of luck).
