A32 gram radioactive sample has a half-life of 2 days how much of the original sample remains after 6 days

First, we will get the "n", the number of half lives, it is the elapsed time over the half life. In the problem, the time is measured in days, so we have 

6/2 = 3 

to get the ending amount of radioactive sample, we have

32g x (1/2)³ = 4 grams of radioactive sample ⇒ the amount left after 6 days