The lifetimes of batteries used in a sensor are normally distributed. A supplier A A\,A sells batteries with a mean lifetime of 1200 hours and a standard deviation of 80 hours.
Find the probability of a battery from supplier A A\,A having a lifetime of less than 1100 hours.
In a shipment of 400 batteries from supplier AAA, find the expected number having a lifetime of less than 1100 hours.
Another supplier B B\,B sells batteries with a mean lifetime of 1250 hours, and it is known that 15% of these batteries have a lifetime of less than 1167 hours.
Find the standard deviation of the lifetimes of batteries from supplier BBB.
Both suppliers sell the batteries for the same price.
State which supplier you would recommend. Give reasons for your answer.
147 exam-style questions on OCR A Level Maths 2.4.6 Probabilities using the normal distribution (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.