Field data for 20 mechanical suspension systems showed 3 failure during the 12 month warranty period. When failure occurred they were repair within 2 days and the system returned to operation. If…

Field data for 20 mechanical suspension systems showed 3 failure during the 12 month warranty period. When failure occurred they were repair within 2 days and the system returned to operation. If the time of failure for the failed system were 4.4 moths, 6.3 months, and 10.7 months after placement in the field, what is the 90% lower confidence estimate on MTBF?