Marisol is considering a cellular phone service plan that is

Marisol is considering a cellular phone service plan that is called “Pick Your Minutes”. Under this plan, she would specify a quantity of minutes, say x, per month that she would buy at 5¢ per minute. Hence, her upfront cost would be $0.05x. If her usage is less than this quantity x in a given month, she loses the minutes. If her usage in a month exceeds this quantity x, she would have to pay 40¢ for each extra minute (that is, each minute used beyond x). (For example, if she contracts for x = 120 minutes per month and her actual usage is 40 minutes, her total bill is $120 ´ 0.05 = $6.00. However, if actual usage is 130 minutes, her total bill would be $120 ´ 0.05 + (130 – 120) ´ 0.40 = $10.00. ) Marisol estimates that her monthly needs are best approximated by the Normal distribution, with a mean of 250 minutes and a standard deviation of 24 minutes. How many minutes should she contract for?

Solution

The question is incomplete i.e. data is there, but the question how many minutes should she contract for, there must be some conditions for the same

Since the actual cost will become 8 times as the book call cost 0.05c and extra call cost is 0.40c

So she must satisfy the 95% confidence interval in order to save the money

z-score for 95% confidence interval = 1.96

1.96 = (number of minutes - mean)/standard deviation

1.96 = (number of minutes - 250)/24

number of minutes = 250 + 24 * 1.96 = 297.05

Hence Marisol should choose the plan for 298 minutes

Please check the question again and reply me back in comments