Answer:
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
