Calculation:

In order to add the given decimals, observe that both the decimal numbers have same sign, so they will get added and the resulting number will carry the negative sign as well.

So first we will write the numbers so that the decimals line up vertically and put zero for the place holders as needed. And then we add the numbers as if they are whole numbers and finally place the decimal point in the sum. So we get

\(\begin{equation}\frac{\begin{array}[b]{r}4.2\\+ 9.3\end{array}}{13.5}\end{equation}\)

Since both the numbers are negative, so the resulting sum will be negative as well.

Thus, the sum of the given decimal numbers is -13.5.