The radioactivity of a certain radioisotope decreases to one-half in 1.0 hour. Three hours later, its radioactivity, compared with that at the beginning of the first hour, is
A) 1/2.
B) 3/2.
C) 1/4.
D) 1/6.
E) 1/8.

Question

Quizplus · Accepted Answer

The radioactivity decreases to one-half in 1 hour, so after 3 more hours (total of 4 hours), it would have halved 4 times (1/2)^4 = 1/16. However, the question mistakenly states "three hours later" but calculates as if it's 4 hours later. The correct calculation for 3 hours later should be (1/2)^3 = 1/8, hence the correct answer is E.

The Radioactivity of a Certain Radioisotope Decreases to One-Half in 1.0