Expected Value question for math

Question

You found a shortcut to school that saves 5 minutes off of taking the freeway. About 1 in every 10 trips you get stuck behind a bus and it ends up taking you 20 minutes longer than the freeway. What is the expected
value of the shortcut?

Answer 1

9 times you'll save 5 minutes each time for a total saving of 9x5=45 minutes

1 time you'll lose 20 minutes.

on net, you gain 25 minutes: 45-20=25

That's for all 10 trips.

For just one trip, you're expected gain is 2.5 minutes

25/10 = 2.5