A residential school has 200 boys and 120 girls. It plans to admit 40 more boys, and the management insists that the ratio of boys to girls must stay exactly what it is now. How many girls must be admitted as well?

The present ratio is 200 : 120 = 5 : 3. With 240 boys the school needs $240\times 3/5=144$ girls, and it already has 120, so 24 more must be admitted. (144 is the total number of girls the school will then have, not the number to be admitted; taking in 40 girls to match the 40 boys would change the ratio to 240 : 160 = 3 : 2.)