Solve 331 + 89: Vertical Addition Practice Problem

$\begin{aligned} &331 \\ +& \\ &~~89 \\ &\underline{\phantom{776}} & \\ \end{aligned}$

To solve this problem, we will perform vertical addition, which involves aligning the numbers based on their place values and adding them column by column, starting from the right.

Step 1: Write the problem in vertical format:
$\begin{array}{c} \phantom{~}331 \\ +\phantom{~}89 \\ \hline \phantom{~} \end{array}$

Step 2: Add the units (rightmost) column:
$1 + 9 = 10$ We write 0 in the units position and carry over 1 to the tens column.

Step 3: Add the tens column including the carry over:
$\begin{aligned} 3 (from ~331) + 8 (from ~89) + 1 (carry over) &= 12. \end{aligned}$ Write 2 in the tens place and carry 1 over to the hundreds column.

Step 4: Add the hundreds column including the carry over:
$3 (from ~331) + 0 (from ~89) + 1 (carry over) = 4.$ Write 4 in the hundreds place.

The result of the addition is:
$\begin{array}{c} \phantom{0}\\ \phantom{~}331 \\ +\phantom{~}89 \\ \hline \phantom{~}420 \end{array}$

Therefore, the solution to the problem is $420$, corresponding to choice 4.