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Which is an equation for the nth terms of the sequence 12,15,18,21

Which system of linear inequalities is represented by the graph?

The random variable x is the number of occurrences of an event over an interval of ten minutes. it can be assumed that the probability of an occurrence is the same in any two-time periods of an equal length. it is known that the mean number of occurrences in ten minutes is 5.3. the appropriate probability distribution for the random variable