1. **State the problem:** Multiply $1000000000000000000000$ by $10000000000000000$. 2. **Use the multiplication rule for powers of 10:** Each number is a power of 10. We can write them as: $$1000000000000000000000 = 10^{21}$$ $$10000000000000000 = 10^{16}$$ 3. **Multiply powers of 10:** When multiplying powers of 10, add the exponents: $$10^{21} \times 10^{16} = 10^{21+16} = 10^{37}$$ 4. **Final answer:** $$1000000000000000000000 \times 10000000000000000 = 10^{37} = 10000000000000000000000000000000000000$$ This is a 1 followed by 37 zeros.