How many uranium-238 atoms are left after 1.338 x 10^10 years? The half-life of uranium-238 is about 4.46 x 10^9 years. How many will there be after 1.338 x 10^10 years? How can I figure this out? I know it's exponential, but how?

First you find the number of half lives in $1.338 \times 10^{10}$ years. That is 3. After every half life, the amount of uranium will halve. So there will be $\frac{1}{2^3}$ as much uranium-238 left.