There is a few main things to remember.
1) The radioactive half-life is the time taken for half the starting material to disappear.
[Starting Amount] ##*## ##[1/2]^n## = Final Amount
Where n is the number of half lives
2) The number of half-lives can be expressed as n = total time / length of half-life)
For example: Lets say the half life of a radioactive substance is 30minutes. If there is 100g of starting material, how much is left after 2 hours.
Answer:
Recognise 2 hours = 4 x 30 minutes, so n=4 half lives
[100g] ##*## ##[1/2]^4## = 6.25g
Worked examples can be found here: ChemTeam Half-Life Problems
