Answer :
[tex]5x-\ girls\\\\
2x-\ boys\\\\
5x+2x=35\\\\
7x=35\ \ \ | divide\ by\ 7\\\\
x=5\\\\
There\ are\ 10\ boys.[/tex]
Set up a proportion:
In a sample of 5:2, you have 7 kids in all.
So the the ratio of boys to kids in a sample of 7 is 2:7.
So set up a proportion:
[tex]\frac{2}{7}[/tex] = [tex]\frac{X}{35}[/tex]
7x =70
X = 10
Therefore, 10 of the boys went walking.