In the seventh grade, the ratio of girls to boys is 7: 5. if there are 21 girls in the seventh grade, how many boys are there?

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! :)

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