Question 1012920
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
{{{(80x + 98)/(x+1) = 83}}}
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