SOLUTION: The ratio of the number of boys to the number of girls in a class of 42 pupils was 4:3 when some boys left the class the number of girls was the same as the number of boys. how man

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Question 1090635: The ratio of the number of boys to the number of girls in a class of 42 pupils was 4:3 when some boys left the class the number of girls was the same as the number of boys. how many boys left the class
Answer by ikleyn(52781) 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 = 4%2F3,

x + y = 42.

Express x via y from the first equation: x = 4y%2F3, and substitute it into the second equation. 
You will get a single equation for the unknown y:

4y%2F3 + y = 42.

Multiply both sides by 4 and simplify.

4y + 3y = 42*3 = 126,

7y = 126, 

y = 126%2F7 = 18.

Now x = 4y%2F3 = %284%2A18%29%2F3 = 24.

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