A power supply consists of three rectifiers in series. Each rectifier has a Weibull failure distribution with = 2.1. However, they have different characteristic

A power supply consists of three rectifiers in series. Each rectifier has a Weibull failure distribution with β = 2.1. However, they have different characteristic lifetimes given by 12,000 hr, 18,500 hr, and 21,500 hr. Find the MTTF and the design life of the power supply corresponding to a reliability of 0.90.