The average score of 100 students taking a statistics final was 70, with a standard deviation of 7. assuming a normal distribution, what is the probability that a student scored 90 or higher

The wording "a student scored 90 or higher" can be interpreted several ways.

A suitable calculator shows that the probability a randomly selected student scored 90 or higher is about 0.0021374.

The probability that at least one student scored 90 or higher is about
  1 - (1 - 0.0021374)^100 ≈ .1926

The probability that exactly one student scored 90 or higher is about
  100*(0.0021374)(1 - 0.0021374)^99 ≈ 0.1729