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Q.
The number of ways of selecting a boy and a girl from a class consisting of $20$ boys and $30$ girls is
J & K CETJ & K CET 2010Permutations and Combinations
Solution:
Number of ways of selecting a boy in a class consisting of 20 boys and 30 girls
$ {{=}^{20}}{{C}_{1}} $
Number of ways of selecting a girl in a class consisting of 20 boys and 30 girls
$ {{=}^{30}}{{C}_{1}} $
Then, the total number of ways of selecting a boy and a girl from a class consisting of 20 boys and 30 girls
$ {{=}^{20}}{{C}_{1}}{{\times }^{30}}{{C}_{1}} $
$ =20\times 30=600 $