Question: In a classroom there are 30 students. 1/3 of them are boys. Among the girls, 40% own dogs, 20% own a cat, and the rest have no pets. How many girls in the class have no pet?
How to solve this?
Answer: 
2/3 students are girls because 1 - (1/3) = 2/3
20 students are girls because 30 x (2/3) = 20
40% of the girls don't have any pets because 100 - 40 - 20 = 40
8 girls in class have no pets because 20 x .4 = 8