i cant get the mean right

A random variable Xfollows the uniform distribution with a lower limit of 730 and an upper limit of 830. a. Calculate the mean and the standard deviation for the distribution (Round intermediate calculation for standard deviation to 4 decimal places and final answer to 2 decimal places.) Answer is complete but not entirely correct. Mean Standard deviation 833.33 28.87 b. What is the probability that X is less than 820? (Round intermediate calculations to at least 4 decimal places and final answer to 2 decimal places.) Answer is complete and correct. Probabiny 0.90