A robot requires one sensor to work. Two sensors are available, one is used by the robot, and one is a spare part. Each sensor fails at rate of lamda. Upon failure the sensor is immediately sent to a shop for repair and the spare one, if working, is used by the robot. The shop repairs one sensor at a time and the time it takes to repair it is exponential with rate u .What is the expected value of the time until both sensors are in for repair given that both are currently in working condition?