Question

if a radioisotope has a half life of 10 years how much of the original material would remain after 40 years

Answer 1

A radioisotope is constantly decaying at a certain rate. How fast or slow? this depends on the particular substance. All known radioisotopes have a half-life, that is the amount of time it takes for half of a substance to decay. Your substance has gone through 4 half lives (40 years total, at 10 years each half-life), so it will have decayed (0.5)(0.5)(0.5)(0.5) = 1/16 or 0.0625 or 6.25 % remaining.