A weighted average multiplies each value by how many times it counts, then divides by the total count. It is not the average of the averages. The plain mean is in mean, median, and mode.
The setup
A class has 20 students averaging 28 and 10 students averaging 22. The combined average is (20 × 28 + 10 × 22) / (20 + 10) = 780 / 30 = 26. Averaging 28 and 22 to get 25 drops the fact that the first group is twice as large.
If you are missing one value, write the equation and solve. Three tests at 30, 28, and x, with an average of 31, means (30 + 28 + x) / 3 = 31. Then x = 35.
What it is not
A percent increase is a different formula, in percent change. “What percent of the group” is in percent problems. Both the weighted-average setup and the percent formulas are on the list in ACT Math formulas.
The bottom line
Multiply, add, then divide by how many items you actually have. Do not average the group averages unless the groups are the same size.
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