Answer: (0.00306,0.00334)
Explanation:
Given : Sample size : n= 420000
The number of users developed cancer of the brain or nervous system =134
Then , the proportion of users developed cancer of the brain or nervous system :

Significance level :

Critical value :

The confidence interval for population proportion is given by :-

Hence, the 90% confidence interval estimate of the percentage of cell phone users who develop cancer of the brain or nervous system is (0.00306,0.00334).