Question

In a room, there are 450 students of which 20% are girls. A group of students have mobile phones with them, of which 40% are girls. Of those students without a mobile phone, only 10% are girls. How many boys have mobile phones in that room?

Answer 1

216 boys

Explanation:

450 students: 20% girls, 80% boys

450*20%=90 girls

450*80%=360 boys

Students having mobile phones: 40% (OF THE 90) girls, 60% (OF THE 360) boys

90*40%=36 girls

360*60%=216 boys

[We honestly don't need to care about the girls without mobile phones as they are only asking how many boys have mobile phones]

Answer 2

90

Explanation:

Total number of students = 450

20 % are girls

So, number of girls =
`20\% * 450`

=
`(20)/(100) * 450`

=
`(1)/(5) * 450`

=
`90`

Now let no. of students having mobile phones be x

So, number of students not having mobile phones = 450-x

Now we are given that A group of students have mobile phones with them, of which 40% are girls

So, This implies 60% of boys out of x having mobile phones.

So, number of girls having mobile phone =
`(40)/(100)x`

Now we are given that Of those students without a mobile phone, only 10% are girls

So, number of girls not having mobile phone =
`(10)/(100)(450-x)`

So, total number of girls :

=
`(40)/(100)x+(10)/(100)(450-x)=90`

=
`4500-10x+40x=9000`

=
`4500+30x=9000`

=
`30x=4500`

=
`x=(4500)/(30)`

=
`x=150`

So, Number of boys having mobile phone =
`(60)/(100) * 150`

=
`90`

Hence there are 90 boys have mobile phones in that room.