From long experience NoFlat Tire knows that the probability

From long experience, NoFlat Tire knows that the probability is .55 that their XB-70 w last 80.000 miles before it becomes bald or fails. An adjustment is made on any tire that does not last 80.000 miles. You purchase four XB-70s. What is the probability only 3 tire will last at least 80.000 miles?

Solution

Prob that tire will last atleast for 80,000 miles = 0.55

Prob that 3 out of 4 will last 80,000 miles = 4C3 * 0.55³ * 0.45¹

                                                                 = 0.299475

                                                                 0.3