1. **State the problem:** Calculate the sum of $1.86 \times 10^9$ and $4.55 \times 10^9$. 2. **Formula used:** When adding numbers in scientific notation with the same exponent, add the coefficients and keep the exponent the same: $$a \times 10^n + b \times 10^n = (a + b) \times 10^n$$ 3. **Apply the formula:** $$1.86 \times 10^9 + 4.55 \times 10^9 = (1.86 + 4.55) \times 10^9$$ 4. **Calculate the sum of coefficients:** $$1.86 + 4.55 = 6.41$$ 5. **Final answer:** $$6.41 \times 10^9$$ This means the sum is $6.41$ billion.