Question 672859
45/100=x/1200
we are solving to see what 45 percent of 1200 is 
make a ratio 45 out of 100 equals what number out of 1200
then you cross multiply
100x=45(1200)
100x=54000
divide both sides by 100
x=540 that is the number of boys there are
take the total 1200 and subtract the 540 boys
1200-540=660
there are 660 girls