Neco 2019 Commerce Question 40

A small business subscribes to a telephone service with a base monthly fee of ₦8,000. Calls are billed as follows: the first 200 minutes each month are charged at ₦45 per minute, the next 300 minutes at ₦35 per minute, and any usage beyond 500 minutes at ₦25 per minute. The business makes 120 calls per month, each lasting an average of \(d\) minutes. With probability 0.3 the total monthly minutes increase by 20 % because of occasional longer calls. The business expects a monthly revenue of ₦2,500,000 and wants its expected telephone expense not to exceed 5 % of this revenue. What is the maximum average call duration \(d\) (in minutes, to two decimal places) that the business can afford while meeting the expense constraint?