He should use a sample size of 655.
We can use the formula
We find the z-value by first subtracting the confidence level from 1:
1 - 0.8 = 0.2
Divide this by 2:
0.2/2 = 0.1
Subtract this from 1:
1 - 0.1 = 0.9
Using a z-table (http://www.z-table.com) we find the z-score associated with this is 1.28.
Now we plug our information into our formula:
We can use the formula
We find the z-value by first subtracting the confidence level from 1:
1 - 0.8 = 0.2
Divide this by 2:
0.2/2 = 0.1
Subtract this from 1:
1 - 0.1 = 0.9
Using a z-table (http://www.z-table.com) we find the z-score associated with this is 1.28.
Now we plug our information into our formula: