The Navy uses a certain type of vacuum tube in a sonar scanning device. Based on past experience, the vacuum tube exhibits the following failure pattern:

Number of Months of Operation Probability of Failure 1 .1 2 .1 3 .2 4 .1 5 .3 6 .2 Failures during operation cost $200 each, but the tube can be replaced before failure for $50. Find the optimal replacement strategy.