A) 
B)
We first find the z-score associated with the level of confidence. For 95% confidence:
Convert 95% to a decimal: 0.95
Subtract from 1: 1-0.95 = 0.05
Divide by 2 (this is the area in the tails): 0.05/2 = 0.025
Subtract from 1 (we don't want the tails, we want the area in the middle): 1-0.025 = 0.975
Using a z-table (http://www.z-table.com) we find the z-score for this is 1.96. We now use the formula
For a 98% confidence level:
Convert 98% to a decimal: 0.98
Subtract from 1: 1 - 0.98 = 0.02
Divide by 2: 0.02/2 = 0.01
Subtract from 1: 1 - 0.01 = 0.99
Using the z-table we see the closest number to this is for the z-score 2.33:
B)
We first find the z-score associated with the level of confidence. For 95% confidence:
Convert 95% to a decimal: 0.95
Subtract from 1: 1-0.95 = 0.05
Divide by 2 (this is the area in the tails): 0.05/2 = 0.025
Subtract from 1 (we don't want the tails, we want the area in the middle): 1-0.025 = 0.975
Using a z-table (http://www.z-table.com) we find the z-score for this is 1.96. We now use the formula
For a 98% confidence level:
Convert 98% to a decimal: 0.98
Subtract from 1: 1 - 0.98 = 0.02
Divide by 2: 0.02/2 = 0.01
Subtract from 1: 1 - 0.01 = 0.99
Using the z-table we see the closest number to this is for the z-score 2.33:
