Operations Management help needed. please check your work, provide explanation if possible. if it's right I'll give you a like.

A Casino has 4 roulette tables. Each table needs to be measured for bias and when they fail the test the wheel must be taken to a shop and cleaned and balanced. Currently each wheel needs to be serviced in this fashion at a rate of about once every 10 days. The servicing process takes an average of about 2 days. The shop employs one tech to fix the machines. The casino is considering buying another wheel because they very much want to have at least three tables available at all times. Currently (with 4 wheels) what is the probability that all four machines are working (choose the nearest answer)? 0.05 0.19 0.40 0.61 0.78 0.84