After two half-lives, how much of the original material has decayed?
25 percent
50 percent
75 percent
100 percent
-
ET

75%

Explanation:

A half-life is the time taken to decay a radioactive material to half of its original weight.  

So if the original material was 100 then after 1 half life it would be half that: 50

Then after another half life it would be 25

So 75% of the original material has decayed.

75% of the material has decayed.

Explanation;

We can start this problem by solving the first half-life assuming the arbitrary condition stating that the initial concentration has a value of 100M, thus, for the first half-life, the concentration dwindles to 50M. Afterwards, the second half-life yields a concentration of 25M, meaning that the 75% of the material has decayed up to the second half-life, of course, the half of the result from the first half-life.

Hope this helps! :)