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Supplementary objective test questions

12.3 Invoices produced within a firm are known to contain errors: 3% contain a very
serious error, 6% a serious error and 12% a minor error.

What is the probability that a randomly-chosen invoice will have a serious
error or a minor error?

A 0.06

B 0.09

C 0.18

D 0.21

12.4 It has been established that 1 professional cyclist out of 100 uses an illegal
substance called 'X'. It is also known that there is a 95% chance that a cyclist
using an illegal substance will test positive in anti-doping tests, and that there is
a 3% chance that a cyclist who is 'clean' would also test positive (false positive).
A professional cycling team is conducting anti-doping tests before a major event
and selects one of its cyclists at random. The cyclist tests positive for the illegal
substance 'X'.

What is the probability that the cyclist does indeed use 'X'?

A 1%

B 5%

C 24%

D 25%

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