Expected value of taking a shortcut. Math help please

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)(5)-1(-20) = 45 -20 = 25 minutes saved in total for all 10 trips

On 9 trips you save 45 minutes, on one trip you lose 20 minutes, For all 10 trips you gain 25 minutes.

Per trip you gain 25/10= 2.5 minutes on average