Question

A scientist kept 100 grams of a radioactive material in a container. After 280 days, he observed that 25 grams of the radioactive material was left in the container. Based on the observation, which of these is the most likely inference drawn by the scientist? (1 point)

A: About 50 grams of the material was left in the container after 70 days.
B: About 50 grams of the material was left in the container after 140 days.
C: The radioactive material will decay totally in 560 days.
D: The radioactive material will decay totally in 840 days.

Answer 1

B: About 50 grams of the material was left in the container after 140 days.

Explanation:

The other person who answered this is correct.

Answer 2

B: About 50 grams of the material was left in the container after 140 days.

Explanation:

Radioactive materials decay at a rate called "half-lives." After a certain amount of time, the grams of radioactive material will be reduced to half. After that same amount of time again, the material will be reduced to half of the half.

1/4 is equal to 1/2 times 1/2. So, when 1/4 of the material remains, two half-lives have elapsed.

The question states that after 280 days, 25 grams of the material was left in the container. 25 grams is 1/4 of the original 100 grams that the scientist started with at time = 0 days.

When 25 grams of material remain, two half-lives have elapsed. 280 days equals two half-lives for this material. So 140 days (half of 280 days) equals one half-life for this material.

After 280 days (two half-lives), one fourth of the material (25 grams) will remain.

After 140 days (one half-life), half of the material (50 grams) will remain.