Question

In a class there are 40 boys and 20 girls. A teacher wants to select class monitor. In how many ways can this be done if either a boy or a girl is selected as a class monitor ?

Answer 1

40c1+ 20C1=
60








way of class monitor

Answer 2

As we know that problems of selection can be solved by combination easily.where
$\rm^{ n}\: C_{r} = \frac{n!}{r!(n - r)!}$

So, Total boys = 40

Girls = 20

Possible ways to choose one boy from 40 boys

$\rm^{ 40}\: C_{1} \\ \\ = \frac{40!}{1!(40 - 1)!} \\ \\ = \frac{40 \times 39!}{39!} \\ \\ = 40$
Possible ways to choose one girl from 20 girls

$\rm^{ 20}\: C_{1}= \frac{20!}{1!(20 - 1)!} \\ \\ = 20$
Thus ways to choose a monitor for class either a girl or a boy
$= 20+ 40 \\ \\ = 60 \: ways$
Hope it helps you