Probability question???

I am struggling even getting started with this problem. Can someone please help me understand.

The probabilities that a batch of 4 computers will contain 1, 2, 3, and 4 defective computers are 0.6274, 0.3102, 0.0575, and 0.001 respectively.

a. Set up a probability distribution to describe this situation.
b. Based on these probabilities how many defective computers would you expect to find in this batch? (This would be the mean of the distribution.)
c. How much would this value reasonably be expected to fluctuate? (This is the standard deviation.)
d. What’s the probability of finding at most 2 defective computers in this batch?

I am having a difficult time with this question can anyone help. The answer is in book however, I want to understand how they came up with this. Thanks

Rise in tides shown here is a frequency distribution for the rise in tides at 30 selected locations in the US.

Rise in tides (inches) Frequency
12.5-27.5 6
27.5-42.5 3
42.5-57.5 5
57.5-72.5 8
72.5-87.5 6
87.5-102.5 3

Find each of these
a. Mean
b. Modal class
c. Variance
d. Standard deviation