Weighted average
In an ordinary mean, every value counts equally. In a weighted average, each value has a weight, as when tests count differently.
A simple example
If 10 has weight 1 and 20 has weight 2, the average is (10 × 1 + 20 × 2) / (1 + 2) = 50 / 3 ≈ 16.7. Divide by the sum of weights, not the number of values.