Probability..ez but problem with the wording.

I have a couple of hours more before this is dued. I actually did complete all 20 questions but my prof said I did this question wrong.

**A chemical engineer is in charge of a particular process at an oil refinery. Past experience indicates that 10% of all shutdowns are dued to equipment failure ***alone*, 5% are dued to a combination of equipment failure and operator error, and 40% involve operator error. A shutdown occurs. Find the probability that

a) equipment failure or operator error is involved;

b) operator error alone is involved;

c) neither operator error nor equipment failure is involved.

Here's what I did:

Let A=equipment failure alone; P(A)=0.1

Let B=operator error; P(B)=0.40

P(AnB) = 0.05

a)P(AUB) = P(A) + P(B) - P(AnB) = 0.45

b)operator error alone = 0.4

c) neither operator error nor equipment failure is involved

= 1-0.45

= 0.55

Prof said it's wrong. Said wordings are tricky. I've been looking at it for the past hour, i know something is wrong but I can't figure out what exactly is wrong.