6.6.4 A system consists of two units, both of which may operate simultaneously, and a single repair facility. The probability that an operating system will fail in a short time interval of length At is u(At) + o(At). Repair times are exponen- tially distributed, but the parameter depends on whether the failure was regular or severe. The fraction of regular failures is p, and the corresponding exponential parameter is a. The fraction of severe failure is q = 1 -p, and the exponential parameter is B