Here is a weighted average calculator. The meaning of weighted average is best explained by the examples given here.
This calculator can also calculate for an individual value in a group needed to make a given weighted average. You
can use it for example to calculate for the final exam score needed to make a certain grade. Please leave
the value to be calculated blank. Click on the add rows button to add more entries. The number of decimal
places for the calculated value can be specified (The default value is 5). This new version also gives the user the
ability to do some row manipulation (insert blank rows, delete rows, move rows).