Just a drop in the bucket

We should first calculate the volume of the ocean: $V = 0.7 \times 4 \pi R^2 d$ (a more correct formula would be $0.7 \times \frac{4}{3} \pi(R^3 - (R-d)^3)$ but we do not need this kind of precision since we are trying to find the order of magnitude). On the other hand, that volume should be made out of raindrops: $N \frac{4}{3} r^3 \pi$ , where $N$ is the number of raindrops. Expressing $N$ from these equations we get $N = 1.7096 \times 10^{26}$ , which has the order of magnitude $26$ .