2011 A-level H2 Mathematics (9740) Paper 2 Question 9 Suggested Solutions

All solutions here are SUGGESTED. Mr. Teng will hold no liability for any errors. Comments are entirely personal opinions.

(i)
(a)
P(faulty)
= P(made by A and faulty) + P(made by B and faulty)
= 0.6(0.05) + 0.4(0.07)
= 0.058

(b)
P(made by A | faulty)
= $\frac{\mathrm{P(made~by~A~and~faulty)}}{\mathrm{P(faulty)}}$
= $\frac{0.6(0.05)}{0.058}$
= $\frac{15}{29}$

(ii)
(a)
P(exactly one of them is faulty)
= $0.058 \times (1 - 0.058) \times 2!$
= 0.109272 (exact)

(b)
P(both were made by A | exactly one is faulty)
= $\frac{\mathrm{P(both~were~made~by~A~and~exactly~one~is~faulty)}}{\mathrm{P(exactly~one~is~faulty)}}$
= $\frac{\mathrm{P(one~is~made~by~A~and~faulty,~the~other~is~made~by~A~and~not~faulty)}}{\mathrm{P(exactly~one~is~faulty)}}$
= $\frac{0.6(0.05) \times 0.6(0.95) \times 2!}{0.109272}$
= $\frac{1425}{4553}$

KS Comments:

Question can be easily solved by drawing a tree diagram. Do take note that we only wrong off NON exact answers to 3sf, so for (iia), we keep the full exact answer.

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KS Teng

KS has been teaching H2/H1 Mathematics and IB mathematics for the more than 10 years. Having taught students from all various Junior Colleges, KS adapts to students' abilities and help them better understand the topics. As someone who loves to teach mathematics and sees it as a truly useful tool in life, KS seeks to enable students to appreciate math. Therefore, his tuition mission is to motivate and cultivate students to be independent and confident thinkers.

2011 A-level H2 Mathematics (9740) Paper 2 Question 9 Suggested Solutions

KS Comments:

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