Two years ago there were 6 grams of a radioactive substance. now there are 5 grams. how much will remain 2 years from now?

Answer

4.444444444444445 grams.

Subtract 2 years' worth of decay from the original amount to find the current amount.
Subtract another 2 years' worth of decay from the current amount to find the amount that will remain after 4 years.