1. **State the problem:** Add the decimals 0.66 and 0.03. 2. **Formula and rules:** To add decimals, align the decimal points and add digits column by column from right to left. 3. **Align decimals:** $$\begin{array}{r} 0.66 \\ +0.03 \\ \hline \end{array}$$ 4. **Add digits:** $$\begin{array}{r} 0.66 \\ +0.03 \\ \hline 0.69 \\ \end{array}$$ 5. **Explanation:** Adding 6 + 3 in the hundredths place gives 9, and 6 + 0 in the tenths place gives 6. 6. **Final answer:** $$0.66 + 0.03 = 0.69$$