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# Uranium-235 (U-235) radioactively decays into Lead-207 (Pb-207).

Uranium-235 (U-235) radioactively decays into Lead-207 (Pb-207). If I'm measuring the number of atoms in a mineral, and I find that there are 1000 atoms of U-235 and 3000 atoms of Pb-207, then how many atoms of these elements were in the mineral when the rock first formed?

Select one:

a. 3000 of U-235 and 1000 of Pb-207

b. 2000 of U-235 and 2000 of Pb-207

c. 4000 of U-235 and 1000 of Pb-207

d. No U-235 and 4000 of Pb-207

e. 4000 of U-235 and no Pb-207

In the last question, I measured 1000 atoms of U-235 (parent) and 3000 atoms of Pb-207 (daughter) in a mineral. If the half life of U-235 is 700 million years, then how old is the mineral? If the mineral came from an igneous rock, then how old is the rock?

Select one:

a. The mineral is 700 million years old, and the rock is the same age

b. The mineral is 1400 million years old, and the rock is the same age

c. The mineral is 700 million years old, and the rock is 1400 million years old.

d. The mineral is 2100 million years old, and the rock is the same age.

e. The mineral is 2100 million years old, but we can't determine the age of the rock.