Question 131553
{{{b}}}= number of boys
{{{g}}}= number of girls
{{{b + g = 561}}}
{{{g = 2b + 15}}}
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Now rearrage the 2nd equation
{{{b + g = 561}}}
{{{2b - g = -15}}}
add the equations
{{{3b = 546}}}
{{{b = 182}}}
{{{b + g = 561}}}
{{{182 + g = 561}}}
{{{g = 561 - 182}}}
{{{g = 379}}}
There are 182 boys and 379 girls
check:364
{{{g = 2b + 15}}}
{{{379 = 2*182 + 15}}}
{{{379 = 364 + 15}}}
{{{379 = 379}}}
and
{{{b + g = 561}}}
{{{182 + 379 = 561}}}
{{{561 = 561}}}
OK