Question 1086282
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A school has 630 students. The ratio of the number of boys to the number of girls is 3:2. 
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{{{system(x+y=630,y/x=3/2)}}}


{{{x+3x/2=630}}}

{{{2x+3x=1260}}}

{{{5x=1260}}}

girls, {{{x=252}}},
boys, {{{y=378}}}




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This ratio changes to 7:5 after the admission of 90 new students.
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b, boys added
g, girls added

{{{system((b+378)/(g+252)=7/5,b+g=90)}}}

{{{5(b+378)=7(g+252)}}}
{{{5b+1890=7g+1764}}}
{{{7g-5b=1890-1764}}}
{{{7g-5b=126}}}
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{{{7g-5(90-g)=126}}}
{{{7g-450+5g=126}}}
{{{12g=126+450}}}
{{{12g=576}}}
{{{g=48}}}------the added girls
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{{{highlight(b=42)}}}-----the added boys