Solve 331 + 89: Vertical Addition Practice Problem

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.