The half-life of uranium-235 is 700 million years. if a rock that formed in the early precambrian era (about 2100 million years ago) originally contained 20 grams of uranium-235, about how much uranium-235 would be left in that rock today?

The answer to your question is  2.5 g of Uranium-235 left

Explanation:

Data

Half-life of Uranium-235 = 700 million years

Total time = 2100 million years

Initial mass = 20 grams

Final mass = ?

Process

1.- Compare the years with the mass left considering the half-life

         2100 million years ago  20 g left

After 700 million years

          1400 million years ago   10 g left

After 700 million years

           700 million years ago     5 g left

After 700 million years

                0 million years ago     2.5 g left