the probability that if you pick only one, it's defective, is 250/6700
Therefore, the probability that one is not defective is 6450/6700
a. You want all 4 to not be defective: (6450/6700)^4
b. all 100 have to be not defective: (6450/6700)^100
If you type this into a calculator, you will get about 0.022, so a probability of 2 % that all of them are not defective. As this is a very small probability, the outlet should plan with returned tires.
