Add:

Explanation

We will write the addition in vertical form and add the digits, column by
 column, working from right to left. We must watch for sums in any place-value column that are greater than 9.

If the sum of the digits in any column is more than 9,we must carry.

To help you understand the process, each step of this addition is explained separately. Your solution need only look like the last step.

 We begin by adding the digits in the ones column:7 8 15. Because 15 1 ten 5 ones, we write 5 in the ones column of the answer and carry 1 to the tens column

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