The half-life of substance-X is 5 minutes. If you start with 20g of substance-X, how much substance-X is leftover after 5 minutes? How about 10 minutes?

At 20g with half-life of 5 minutes there would be 10g leftover in 5 minutes. In 10 mins, there would be 5g leftover. Half-life is actually what it says, it refers to time it takes for half of the active elements, etc to break down at which time the potency of the "product" is half as strong. This term is used mainly with radioactive items or medicines.