Since nothing discourages return visits more than long lines, ski resorts have to move people up the mountain quickly. The lift systems they use need to be reliable. This past summer, one resort tested the reliability of their $20$ different lifts by using a time$-$terminated test. The test ran for $100$ hours of continuous operation. During the test, four lifts broke down, at $40,65,80,$ and $89$ hours. Calculate the mean life for the lifts. If it takes an average of $12$ hours to repair a lift, calculate the availability of the lifts.