a) In order to calculate what percent of the machine parts manufactured were defective, you proceed as follow:
divide the defective machine between the total number of machines:
9/500 = 0.018
next, you multiply the previous result by 100:
0.018 x 100 = 1.8%
Hence, 1.8% of the total number of machines were defective
b) In this case you have:
subtract total machine and defective ones:
500 - 9 = 491
divide the previoues result by the number of toal machine:
491/500 = 0.982
multiply the previoues result by 100:
0.982 x 100 = 98.2
Hence, 98.2% of the total number of machine were NOT defective.