SOLUTION: Mark's average (arithmetic mean) on four tests is 75. What grade does he need on his fifth test to raise his average to an 80? I need help on this question from an online assig

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Question 294484: Mark's average (arithmetic mean) on four tests is 75. What grade does he need on his fifth test to raise his average to an 80?
I need help on this question from an online assignment, and it ends tonight so please hurry to answer this
Thank you
Found 2 solutions by Fombitz, richwmiller:
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
1.A1=%28J%2BK%2BL%2BM%29%2F4=75
2.A2=%28J%2BK%2BL%2BM%2BN%29%2F5=80
where J,K,L,M are the grades from the first 4 tests, N is the next test grade.
From eq.1,
J%2BK%2BL%2BM=300
Use that in eq.2,
J%2BK%2BL%2BM%2BN=400
300%2BN=400
N=100
He needs to get 100 on his next test.

Answer by richwmiller(17219) About Me  (Show Source):
You can put this solution on YOUR website!
I was about to answer this out of turn, but then I saw the ending " please hurry to answer this."
You do realize we are volunteer tutors. We don't get paid.
Why should we put your problem ahead of the others who were here first?
I will give you a few hints.
What was the total of his first 4 tests?
Add that to his new average (x) and divide by 4+1
What will you set it equal to?
A second way
What will the new total be if he gets an average of 80 on 5 tests?

A third way
Add up every time he missed 80 on the first four tests and add that to 80 for the grade he has to get on the fifth test.
Hint: He isn't going to do it. Possible but not likely.