Per hour, he snores 3 times per hour since he snores once every 20 minutes. In 3 hours, there are 180 minutes, so 180/20 = 9. The dog snores 9 times in 3 hours.
Using z-scores for 130 and 140 hours, and a standard normal distribution table, the cumulative probabilities for 130 and 140 hours are 0.254 and 0.673 respectively.
The required probability is 0.673 - 0.254 = 0.42 to 2 d.p.
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