the half-life of carbon-11 is 20 minutes. How long will it btake for 600g of carbon-11 to decay to 30g?

Answer 1

To 1/20?

1/20=e^(-.692t/thalf)
ln .05=-.692t/20min

I get about 87 min. gross check. That is about 4 half lives, or (1/2)^4= 1/16 about .05, check.

Answer 2

The half-life of carbon-14 is 5730 years. If a sample contains 15 milligrams of
carbon-14, about how much carbon-14 was in the sample 15,000 years ago?