Question 1175774
Before change,
5g boys, g girls


p boys and p girls joined.
5g+p boys, g+p girls;
{{{system(5g+p=2(g+p),g+p=12)}}}



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{{{g=12-p}}}
-
{{{5(12-p)+p=2g+2p}}}
{{{60-5p+p=2(12-p)+2p}}}
{{{60-4p=24-2p+2p}}}
{{{60-4p=24}}}
{{{15-p=6}}}
{{{highlight(p=9)}}}------------9 girls joined