Statistics
posted by Tom .
Determine the sample size necessary to estimate a population proportion to within .03 with 90% confidence assuming you have no knowledge of the approximate value of the sample proportion.

Try this formula:
n = [(zvalue)^2 * p * q]/E^2
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 .03 in the problem. Zvalue is found using a ztable to represent 90% interval. Round final result to next highest whole number.
Also: ^2 means squared and * means to multiply.
I hope this will help get you started. 
n = [(zvalue)^2 * p * q]/E^2