1. **State the problem:** Add the three decimal numbers: $5.78$, $3.57$, and $6.6$. 2. **Recall the addition rule for decimals:** Align the decimal points and add digits column by column from right to left. 3. **Align the numbers:** $$\begin{array}{r} 5.78\\ 3.57\\ 6.60\end{array}$$ 4. **Add the hundredths place:** $8 + 7 + 0 = 15$. Write $5$ and carry over $1$ to the tenths place. 5. **Add the tenths place:** $7 + 5 + 6 = 18$, plus the carried over $1$ equals $19$. Write $9$ and carry over $1$ to the units place. 6. **Add the units place:** $5 + 3 + 6 = 14$, plus the carried over $1$ equals $15$. Write $5$ and carry over $1$ to the tens place. 7. **Add the tens place:** Only the carried over $1$ remains, so write $1$. 8. **Combine all results:** The sum is $15.95$. **Final answer:** $15.95$