Final answer:
After 140 years, which is equivalent to two half-lives of uranium-232, one quarter of the original amount remains; therefore, from an initial 40 milligrams, 10 milligrams of uranium-232 will remain.
Step-by-step explanation:
The question involves the concept of half-life, which is part of nuclear physics and chemistry. To find out how much uranium-232 will remain after 140 years, given it has a half-life of 70 years, we simply calculate the number of half-lives that will have passed in 140 years.
140 years is exactly two half-lives of uranium-232 (since 70 years is one half-life). After one half-life, half the initial amount remains, so after 70 years, you would have 20 milligrams left from the original 40 milligrams. After the second half-life, half of this remaining amount will still be present. So after another 70 years (making a total of 140 years), you would have half of 20 milligrams, which is 10 milligrams.
In summary, after 140 years, you can expect to have 10 milligrams of uranium-232 remaining from the initial 40 milligrams you started with.