P(1≤x≤3) defined the probability that the result is between 1 and 3, included. You can answer this question in two ways:
1. Sum the probabilities of good events:
From the graph, we have
So,
2. Use complementary probabilities
Asking that the result is 1, 2 or 3 is the same as asking that the result is not 4. The probability that the result is 4 is 0.16, so the probability that the result is not 4 will be
The result is obviously the same.