Add: 421 + 123 + 245

Explanation

We will write the addition in vertical form with the ones digits in a  column, the tens digits in a column, and the hundreds digits in a column. Then we will add the digits, column by column, working from right to left. 

Like money, where pennies are only added to pennies, dimes are only added to dimes, and dollars are only added to dollars, we can only add digits with the same place value: ones to tens, hundreds to hundreds.

Solution

We start at the right and add the ones digits, then the tens digits, and finally the hundreds digits and write each sum below the horizontal bar.

\[ \begin{array}{r@{}r@{}r@{}r} \color{blue}{} & \color{blue}{} & \color{blue}{} & \text{} \\ & 4 & 2 & 1 \\ + & 1 & 2 & 3 \\ + & 2 & 4 & 5 \\ \hline & 7 & 8 & 9 \\ \end{array} \]

If an addition of the digits in any place value column produces a sum that is greater than 9, we must carry.