In a class, there are 30 boys who are over 160 cm tall. If these constitute three-fourths of the boys and the total number of boys is two-thirds of the total number of students in the class, what is the number of girls in the class ?
Choose the correct answer:
A) 6 B) 12 C) 18 D) 24 E) 20
Explanation:   Let the number of boys be x. Then, (3/4)x = 30 or x = 30 x(4/3) = 40.   If total number of students is y, then (2/3) y = 40 or y = 40 x (3/2) = 60.   Therefore Number of girls in the class = (60 - 40)= 
<gadget id="calculator">(60 - 40)</gadget>
<output>20</output>
20.   Answer: 20
<result>E</result>
In a class, there are 30 boys who are over 160 cm tall. If these constitute three-fourths of the boys and the total number of boys is two-thirds of the total number of students in the class, what is the number of girls in the class ?