One-tenth of the students at a nursery school are 4 years old or older. If 20 students have not yet reached their third birthday, and a total of 50 students are not between 3 years old and 4 years old, how many children are in the nursery school? Choose the correct choice from the following answers:
A) 150
B) 300
C) 450
D) 490
E) 620

Rationale: x / 10 students are > 4 yrs 20 students are < 3 yrs x / 10 + 20 = 50 x / 10 = 30 x = 300 answer : b
Answer: B