So we are given the fraction ratio...
7 girls
---------
5 boys
if we make the fraction
21 girls
----------
x boys
equal each other, we can cross multiply and find x.
Cross multiplying means to multiply 7 times x and 21 by 5.
then we set up a little equation
7x = (21)(5) First lets solve what 21 times 5 is
7x = 105 Now we have to divide both sides by 7 to get x alone.
÷7 = ÷7 On the left side of the equation the 7 cancels and we solve on the right.
x = 15
So we know that when we have 21 girls in the 7th grade classroom we also have 15 boys.
Good Luck! :)
<3
