Step 1:

Total number of Danish cell phone users in the survey are $\"\"$ .

The probability of phone users suffered from brain cancer or nervous system $\"\"$ .

Definition of specific confidence interval for a proportion : $\"\"$ ,

where $\"\"$ is the population proportion.

$\"image\"$ is the sample proportion.

$\"image\"$ .

n is the sample size.

$\"\"$ is the confidence interval.

Step 2:

$\"image\"$ for 90 % confidence interval is 1.65.

$\"\"$ .

90% of confidence interval for the proportion is

$\"\"$