All the FI's I've spoken to agreed that they would never send a student off to an airfield with a thunderstorm sat over it. It is the most ridiculously long question but all the pointers were stay on the ground and fly another day, not take-off and take a chance.
Gemma I think you are missing the point of the Exams. The examinations are not based on what an instructor would do with a student. Think about the situation in say five years time when you are at an airfield in the middle of nowhere and you need to get back home for work the next day. People do alarming stupid things in aircraft which may not be apparent to you at the moment but you wait, you will be posting on here one day "Why on earth did he do that"?
The problem in light aviation is not the exams it is the pilots that think the exams are a problem.
By the way, instructors should not be just telling you they would not be sending students to an airfield with a thunderstorm sitting over it, what they should be saying is they would not be sending anyone on a solo cross country with an unstable airstream with a high percentage chance of showers and /or thunderstorms. Showers and thunderstorms are produced by the same unstable airstreams but it is showers that catch out students (and instructors) more so than thunderstorms. It can be 8/8ths blue at the home airfield and at the destination but a series of showers enroute can turn a VFR flight into a instant IFR flight with dire consequences for students.
The CAA answer as indicated in one of the above posts is above is correct.
This is am arithmetical calculation and problem, treat is as such and one, you will get the correct answer and two you will understand the purpose of the exam.