ProbabilityHardBloom L3
Question
A letter is to come from either LONDON or CLIFTON. The postal mark on the letter legibly shows consecutive letters 'ON'. What is the probability that the letter has come from LONDON.
Options
A.$\frac{12}{17}$
B.$\frac{11}{17}$
C.$\frac{5}{17}$
D.$\frac{13}{17}$
Solution
Sol. [Key Concept: Letter from LONDON / CLIFTON: condition on the source, compare letter frequencies.]
$$P = \frac{\frac{1}{2} \times \frac{2}{5}}{\frac{1}{2} \times \frac{1}{6} + \frac{1}{2} \times \frac{2}{5}} = \frac{12}{17}$$
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