It is remarkable that a science, which began with the consideration of
games of chance, should be elevated to the rank of the most important
subject of human knowledge. -Pierre Simon Laplace

Where a mathematical reasoning can be had, it is as great a folly to
make use of any other, as to grope for a thing in the dark, when
you have a candle in your hand.
- JOHN ARBUTHNOT

If you study Six Sigma, you work with probabilities very often. Examples of basic questions -

• What is the probability of defects in process A or in process B or in Process C if the probabilities in A, B and C are 0.2, 0.3, 0.1 respectively?

• What is the probability of defects in this process? If someone says that probability is 0.35, is it a small chance or a worrisome probability?

To refresh such concepts, you can go through some content on probability.

• Probability-1
• Probability-2
• Probability-3

You can also learn these basics using on-line resource by clicking here