The length of time a system is down (that is, broken) is described (approximately) by the probability

The length of time a system is €œdown€ (that is, broken) is described (approximately) by the probability distribution in Table 7.6.2. Assume that these downtimes are exact. That is, there are three types of easily recognized problems that always take this long (5, 30, or 120 minutes) to fix.
a. What kind of probability distribution does this table represent?
b. Find the mean downtime.
c. Find the standard deviation of the downtime.
d. What is the probability that the downtime will be greater than 10 minutes, according to this table?
e. What is the probability that the downtime is literally within one standard deviation of its mean? Is this about what you would expect for a normal distribution?

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TABLE 7.6.2 Probability Distribution of Downtime Problem Probability Downtime (minutes) Minor Substantial Catastrophic 20 0.60 0.30 0.10 30