Long answer·Hard exam skill (4/5)·No calculatorNeeds mentor review
Difficulty guide
The label describes the problem, not your ability.
1/5 - Foundational
Warm-up setup; checks one basic idea.
2/5 - Routine exam skill
Normal syllabus skill; usually one main rule or interpretation.
3/5 - Standard multi-step
Exam-style problem with multiple steps or a common trap.
4/5 - Hard exam skill
High-end exam problem; combines methods carefully.
5/5 - Challenge
Stretch problem beyond normal exam pressure.
Question
Three warning systems work independently during a drill. Their working probabilities are and respectively.
Total: 5 marks
(a) Find the probability that all three systems work. (2 marks)
(b) Find the probability that at least one system fails. (2 marks)
(c) Find the probability that only system works. (1 mark)
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