SOLUTION: mark's average score after x scores is 80%. He calculate he will have an 83% if he scores 98% on the next test. How many tests has mark attempted so far?

Question 1012920: mark's average score after x scores is 80%. He calculate he will have an 83% if he scores 98% on the next test. How many tests has mark attempted so far?
Found 2 solutions by Boreal, fractalier:
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
Average =total/n
80=total/n
83=(total+98)/(n+1)
80n=total
83n+83-98=total=83n-15
Therefore, 80n=83n-15
-3n=-15
n=5
He has taken 5 tests so far,and total score is 400 for an average of 80
with 6 tests,the total will be 498, and the average is 83.
Answer by fractalier(6550) (Show Source): You can put this solution on YOUR website!
Call x the number of tests taken so far.
Thus his total points (assuming 100 points per test) is 80x.
If he scores 98 on the next one, his total will be 80x + 98.
If we divide that by x+1 tests, he'll average 83...we write that as

We cross-multiply and get
83(x+1) = 80x + 98
Now solve and get
83x + 83 = 80x + 98
3x = 15
x = 5 tests so far
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