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 25 students are not between 3 years old and 4 years old, how many children are in the nursery school?  Choose the correct choice:
A) 51
B) 20
C) 40
D) 50
E) 55

Rationale: "x / 10 students are > 4 yrs 20 students are < 3 yrs x / 10 + 20 = 25 x / 10 = 5 x = 50 answer : d"
Answer: D