Before the excavators were purchased, a reliability engineer did some reliability block modelling of the sub-system comprising the bucket, teeth and a lift cylinder system and a hydraulic pump. He/she assumed each component lifetime has an Exponential distribution. Assuming the MTBF is 15000 hours for the bucket, 900 for the teeth, 10000 for the lift cylinder system and 12000 for the hydraulic pump. What is the reliability for the sub-system over one month (450 hour) period? [The subsystem fails if one of the components fails.]