After 5730 years you have half of what you started with. Say you started with 100 so now you have 50. Stop
Now we begin with 50. After 5730 we have half of that we started with ergo 25.
Combining experiment 1 and 2 says that after two halflifes 11460 years there are 25 left.
Halflife is the time needed for half of all the isotopes to change into something else.
If it doesn't make sense yet then I will be happy to explain some more. Or maybe tuppence will. By the way tuppence, what was your post referring to? I don't really understand.