How much of the original element would be present after three half-lives?

Answer 1

Half-life is found to be a characteristic feature of a radioactive element. If a sample of radioactive element contains N nuclei at a time (taken as zero), then after a time equal to half-life, the number reduces to N/2; during the next half life that is after two half-lives N/4 and after three half-lives N/8 and so on.
For example half-life of radium-226 is 1622yrs. Suppose we start with 1mg of radium, in 1622 years it reduces to 0.5mg, 0.25mg in 3244yrs, 0.125mg in 4866yrs and so on.

Answer 2

1/2 * 1/2 * 1/2 = 1/8 of the original

Answer 3

1/2 of the original element is lost during each half-life (hence the name).

1/2 * 1/2 * 1/2 = 1/8 or 12.5%

Answer 4

After 1 half-life there's 1/2
After 2 half-lives there's 1/4
After 3 half-lives there's 1/8

Answer 5

one eighth (1/8). 1/2 times 1/2 times 1/2 equals 1/8