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Question
There are 10 professors and 20 students out of whom a committee of 2 professors and 3 students is to be formed. Find the number of ways in which this can be done. Further find in how many of these committees:
a particular professor is included.
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Solution
Clearly, 2 professors and 3 students are selected out of 10 professors and 20 students, respectively.
Required number of ways =\[{}^{10} C_2 \times^{20} C_3 = \frac{10}{2} \times \frac{9}{1} \times \frac{20}{3} \times \frac{19}{2} \times \frac{18}{1} = 51300\]
If a particular professor is included, it means that 1 professor is selected out of the remaining 9 professors.Required number of ways =\[{}^{20} C_3 \times^9 C_1 = \frac{20}{3} \times \frac{19}{2} \times \frac{18}{1} \times \frac{9}{1} = 10260\]
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