15.13 In an experiment to investigate the performance of four different brands of spark plugs intended for use on a 125-cc motorcycle, five plugs of each brand were tested, and the number of miles (at a constant speed)

until failure was observed. A partially completed ANOVA table is given. Fill in the missing entries, and test the relevant hypotheses using a .05 level of significance. (Hint: See Table 15.2.)

Source of Variation df Sum of Squares Mean Square F Treatments Error 235,419.04 Total 310,500.76