SOLUTION: The ratio of boys to girls in a class is 3:4.I f there are 42 pupils in class, how many are girls?

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Question 1006067: The ratio of boys to girls in a class is 3:4.I f there are 42 pupils in class, how many are girls?
Answer by ikleyn(52787) About Me  (Show Source):
You can put this solution on YOUR website!
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Let x = be the number of boys in the class, and y be the number of girls.

Then you have two equations:

x%2Fy = 3%2F4,

x + y = 42.

Express x via y from the first equation: x = 3y%2F4, and substitute it into the second equation. 
You will get a single equation for the unknown y:

3y%2F4 + y = 42.

Multiply both sides by 4 and simplify.

3y + 4y = 42*4 = 168,

7y = 168, 

y = 168%2F7 = 24.

Now x = 3y%2F4 = %283%2A24%29%2F4 = 18.

Answer. There are 18 boys and 24 girls in the class.