A burr is a thin ridge or rough area that occurs when shaping a metal part. These must be removed by hand or by means of some newer method such as water jets, thermal energy, or electrochemical processing before the part can be used. Assume that a part used in automatic transmissions typically averages two burrs each. What is the probability that the total number of burrs found on seven randomly selected parts will be at most four? (Based on "Advances in Deburring" by B. Hignett, Production Engineering, December, 1982, pp. 44-47.)