You want to estimate the grades of st2%atistics students who gets grades of C or better. How many students should you survey to be 95% confident that the sample percentage is off by no more than 2 percentage points.
Try this formula:
n = [(z-value)^2 * p * q]/E^2
With your data:
n = [(1.96)^2 * .5 * .5]/.02^2
(Finish the calculation and round to the next highest whole number.)
Note: n = sample size needed; .5 for p and .5 for q are used if no value is stated in the problem. E = maximum error, which is .02 (2%) in the problem. Z-value is found using a z-table (for 95%, the value is 1.96).
I hope this will help.