ProbabilityHardBloom L3
Question
If the papers of 4 students can be checked by any one of the 7 teachers. Then the probability that all 4 papers are checked by exactly 2 teachers is
Options
A.$\frac{18}{343}$
B.$\frac{12}{343}$
C.$\frac{2}{49}$
D.$\frac{6}{49}$
Solution
Sol. [Key Concept: Papers checked by any teacher: count favorable assignments.]
$$P = \frac{\ ^{7}C_{2}\left( 2^{4} - 2 \right)}{7^{4}} = \frac{6}{49}$$
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