Potassium-40 has a half-life of 1.3 billion years. If a rock was determined to be around 3.9 billion years old what percentage
of the original potassium-40 would be left in that rock sample?

The 3.9 billion year old rock has undergone three half-lives (3.9 divided by 1.3 = 3). After 3 half-lives, 12.5% of the potassium-40 remains undecayed.

i think that c)gold was found there

but i am not sure

can you give me more to work here with plzs and you