1. **State the problem:** We need to find the sum of 309.648 and 84.93. 2. **Formula used:** Addition of decimal numbers is done by aligning the decimal points and adding corresponding digits. 3. **Align the numbers:** $$\begin{array}{r} 309.648 \\ +\ 84.930 \\ \hline \end{array}$$ 4. **Add the numbers digit by digit starting from the right:** - Thousandths place: $8 + 0 = 8$ - Hundredths place: $4 + 3 = 7$ - Tenths place: $6 + 9 = 15$ (write 5, carry 1) - Units place: $9 + 4 + 1 = 14$ (write 4, carry 1) - Tens place: $0 + 8 + 1 = 9$ - Hundreds place: $3 + 0 = 3$ 5. **Write the final sum:** $$309.648 + 84.93 = 394.578$$ This is the sum of the two numbers.