Question 1
The radioisotope potassium-40 decays to argon-40 by positron emission with a half-life of 1.3 × 10
9 yr. A sample of moon rock was found to contain 78 argon-40 atoms for every 22 potassium-40 atoms. The age of the rock is
◦ 8.1 × 10
–10 yr
◦ 2.4 × 10
9 yr
◦ 2.8 × 10
9 yr
◦ 4.6 × 10
9 yr
◦ 6.8 × 10
9 yr
Question 2
The Rb-87/Sr-87 method of dating rocks is often used by geologists:

t
1/2 = 6.0 × 10
10 yr
Estimate the age of a rock sample in which the present-day mole ratio of Rb-87 to Sr-87 is 36:1.
◦ 2.4 × 10
9 yr
◦ 1.7 × 10
9 yr
◦ 3.1 × 10
11 yr
◦ 4.1 × 10
–11 yr
◦ 3.6 × 10
11 yr