With a distribution of 10 scores, nine of them are 3, 5, 9, 1, 9, 2, 0, 3, and 9. The last one is greater than 5 but it is not 9. Is it possible to determine the mean of the distribution?
I like Kerrie's answer, as long as the scores are discrete, integer data. It appears you have discrete, integers. If that is not the case, then read on:
If the missing score can be any number greater than 5 and less than 9, then plug 5 and 9 into Kerrie's equations intead of 6 and 8; also change the ≤ signs into < signs. This only applies if any score between 5 and 9 is possible (continuous data).
If the data are discrete, but not necessarily integers, then use Kerrie's method but use the nearest discrete data points instead of 6 and 8. For example, if the scores can be 5.0, 5.1, 5.2, 5.3..., then plug in 5.1 and 8.9. Keep the ≤ signs.