You would definitely use Bidmas.
To be honest, I have never taught my pupils more complex than if x = 3, what is 6x + 5, or if x is 4.5, what is 2x + 5, so I wouldn't stress on this.
In fact, what little time you have left, should be spent talking exam tactics rather than specific questions. Basically tell her to go to the toilet in the break, keep filling in the boxes, blank out each section when you move on to the next and, above all, keep your nerve.
Regard it as a decathlon in which you can afford to be useless at javelin, shot putting and hurdles, okay at discus etc. etc, and really good at sprinting and high jump, and still win the bronze medal (I say bronze because it's the same prize whether you score 330 or 430, as far as Camp Hill Girls is concerned).