There's an ad on TV for a news-entertainment show saying there is a 40% chance you have a bought a product containing palm oil. Let's assume they are talking about people buying food products at the supermarket. And let's say they have determined that 40% of food products at the supermarket contain palm oil. If you bought one product you would have 40% of buying a product containing palm oil.

Presumably buying more food products increases one's chances of buying a product containing palm oil. How is this represented mathematically? In 6 years of secondary school maths (including double maths in final years) I always found the translation of real world probability situations into the appropriate equations of permutations and combinations one of the most unclear areas. (Now my problem is I can't even remember all the basic equations and when to apply them). The equations are simple but there application can be tricky.

Please shine some light, mathemagicians.