Question 1090635
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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/y}}} = {{{4/3}}},

x + y = 42.

Express x via y from the first equation: x = {{{4y/3}}}, and substitute it into the second equation. 
You will get a single equation for the unknown y:

{{{4y/3}}} + y = 42.

Multiply both sides by 4 and simplify.

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

7y = 126, 

y = {{{126/7}}} = 18.

Now x = {{{4y/3}}} = {{{(4*18)/3}}} = 24.

<U>Answer</U>. There are 24 boys and 18 girls in the class.
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