Question 1057815
This is my approach if you want dependent and
independent variables.
I'll plot number of students in the class on the 
vertical axis and call it {{{ y }}},  the dependent variable
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I'll plot {{{ x }}}, the number of boys on the horizontal axis
which is the independent variable
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{{{ x + 4 }}} is the number of girls
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{{{ y = x + x + 4 }}}
Here's the system of equations:
(1) {{{ y = 2x + 4 }}}
(2) {{{ y = 26 }}}
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Here's a plot of (1) and (2):
{{{ graph( 400, 400, -3, 36, -3, 36, 26, 2x + 4 ) }}}
As you can see, {{{ x = 11 }}} looks like the solution
There are 11 boys and {{{ x+4 }}} = 15 girls