Answer:
it saves 2.5 minutes
Step-by-step explanation:
The expected value of a random variable is the sum of products of possibilities and the probability of each.
Here, you have a probability of 0.9 of saving 5 minutes, and a probability of 0.1 of adding 20 minutes.
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The expected value of the shortcut is ...
0.9(-5) +0.1(20) = -4.5 +2.0 = -2.5
The shortcut is expected to save 2.5 minutes on average.
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Additional comment
The probability of 0.9 comes from the 9 of 10 trips that result in a time saving.
