Answer:
about 4.8%
Step-by-step explanation:
You want to know the probability of a release greater than 3000 ppm, given that releases are normally distributed with a mean of 2800 ppm and a standard deviation of 120 ppm.
The probability can be found using a spreadsheet or statistical calculator to evaluate the normal CDF function.
P(X > 3000) ≈ 0.04779 ≈ 4.8%
The probability of interest is about 4.8%.
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Additional comment
The attachment shows the probability to as many significant figures as you may want. We have rounded to 2 significant figures, because the problem numbers have that precision.