Astronomy, age of a moon rock, half life

[CrossFit415]

Moon rocks containing uranium 238, decays to lead with a half life of 4.5 billion yrs. The lunar rock sample you're invistigating contains half of the original uranium 238. How old is the rock?

How would I go about solving this? What formula should I use?

 

[zhermes]

What is a half-life? What does half-life mean?