How do I Adjust Dynamic Weighted Averages?

Question

I am trying to create a formula in Excel that tries to create a weighted average score for a person. The only problem is that sometimes one or two of the components of the weighted average may not be relevant for a person. My question is, how do I re-weight each component for them each to be mathematically correct?

For instance:

Grade:                 50% Weighting
Attitude:              30% Weighting
Email Responsiveness:  10% Weighting
Timeliness             10% Weighting

So, what happens when Email Responsiveness can no longer be a part of the weighting because no emails were ever even sent out to this person? How would I readjust the weightings so that only Grade, Attitude, and Timeliness are factored into the weighted average and so that they maintain their relative importance?

Answer 1

$$ \frac{(0.5)\mbox{grade}+ (0.3)\mbox{attitude}+(0.1)\mbox{timeliness}}{0.9} $$