Answer: quickstart batteries will need to be guaranteed for 26 months
Explanation:
the question is incomplete to calculate guarantee period given the maximum refund percentage of 7% we need the mean average life of Quick start batteries and a standard deviation. Let us assume that the mean average life of quickstart batteries is 36 months and standard deviation is 7 months.
P(Z < z*) = 0.07 has a z* = -1.48 [from the Standard Normal table]
Answer = 36 + (-1.48)(7) = 25.64 = 26 months or approximately 35 months
