Answer:
A sample of 499 is needed.
Step-by-step explanation:
In a sample with a number n of people surveyed with a probability of a success of
, and a confidence level of
, we have the following confidence interval of proportions.

In which
z is the zscore that has a pvalue of
.
The margin of error is given by:

In this question, we have that:

90% confidence level
So
, z is the value of Z that has a pvalue of
, so
.
How large a sample would be required in order to estimate the fraction of tenth graders reading at or below the eighth grade level at the 90% confidence level with an error of at most 0.03
We need a sample of n, which is found when
. So






Rounding up
A sample of 499 is needed.