Math CentralQuandaries & Queries


Question from Cindy, a parent:

We need to weight numbers differently in terms of a percentage calculation. One number {80 for example} at 65% and another number {100 for example} at 35%. How do we calculate these into one total grade for example.

Hi Cindy,

You should probably ensure that the two grades are on the same basis. So, for example suppose the score on the first assignment was 80 out of a possible 90 and the score on the second assignment was 100 out of a possible 120. To scale each of them so they are out of a possible 100 I would do the following

  1. 80 out of 90 becomes 80/90 ×100 = 88.89

  2. 100 out of 120 becomes 100/120 ×100 = 83.33

The first assignment is weighted 65% 0r 0.65 and the second assignment is weighted 35% or 0.35 so the weighted sum, out of a possible 100 points is

88.89 × 0.65 + 83.33 × 0.35 = 86.95

I hope this helps,

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