Chapter 3.4A, Problem 6A

Janet worked her addition problems by placing the partial sums as shown here:

$\begin{array}{l}569\\ \underset{\_}{+645}\\ 14\\ 10\\ \underset{\_}{11}\\ 1214\end{array}$

a. Use this method to work the following addition problems.

a.

$\begin{array}{l}687\\ \underset{\_}{+549}\end{array}$

b.

$\begin{array}{l}359\\ \underset{\_}{+673}\end{array}$

b. Explain why this algorithm works.