Everyday Maths

Two group averages of 80 and 50 do not make 65 if the groups are different sizes. Here is the worked example where the two methods disagree by nine points.

It is roughly the typical distance from the mean. Here is how to compute one by hand, why there are two divisors, and why the number means nothing on its own.

One line of algebra turns your current standing and the final's weight into the score you need. And if that score comes out above 100, the honest answer is to say so.