The ratio of number of boys and girls in a school 720 students is 7 : 5. How many more girls should be admitted to make the ratio 1 : 1 ?

Answer: Option B

Number of a boy in the school = 720 × $$\frac{7}{12}$$   = 420
Number of a girl in the school = 720 × $$\frac{5}{12}$$   = 300
To maintain 1 : 1 we will have to add number of girls = 420 - 300 = 120