Probability on the job
Probability is favourable outcomes over total outcomes, and independent events happening in a row multiply.
A worked example
If 8 of 400 parts are defective the rate is 8/400 = 2%, so a shipment of 1 500 parts would be expected to contain 0.02 x 1 500 = 30 defective ones.
The mistake people actually make
Odds compare successes to failures, so odds against of 5 : 3 mean a probability of 3/8. A run of good results does not make a failure more likely on the next independent trial.