1. **State the problem:** Add the three decimal numbers: $5.78$, $3.57$, and $6.6$. 2. **Formula used:** To add decimals, align the decimal points and add digit by digit from right to left. 3. **Align the numbers:** $$\begin{array}{r} 5.78 \\ 3.57 \\ 6.60 \\ \hline \end{array}$$ 4. **Add the hundredths place:** $8 + 7 + 0 = 15$. Write $5$ and carry over $1$. 5. **Add the tenths place:** $7 + 5 + 6 = 18$, plus the carryover $1$ equals $19$. Write $9$ and carry over $1$. 6. **Add the units place:** $5 + 3 + 6 = 14$, plus the carryover $1$ equals $15$. Write $15$. 7. **Combine the results:** The sum is $15.95$. **Final answer:** $5.78 + 3.57 + 6.6 = 15.95$