We start by writing what we are converting, which is 180 mg/L, as a ratio. Then, we need to convert the milligrams into grams and the liters into deciliters, converting one unit at a time. We will start by converting the milligrams into grams. Then, we will convert the liters to deciliters. $$\begin{aligned} 180\,\frac{\text{mg}}{\text{L}}\amp=\frac{180\,\text{mg}}{1\,\text{L}}\\ \amp=\frac{180\,\text{mg}}{1\,\text{L}}\cdot\frac{1\,\text{g}}{1000\,\text{mg}}\cdot\frac{1\,\text{L}}{10\,\text{dL}}\\ \amp=\frac{180\,\cancel{\text{mg}}}{1\,\cancel{\text{L}}}\cdot\frac{1\,\text{g}}{1000\,\cancel{\text{mg}}}\cdot\frac{1\,\cancel{\text{L}}}{10\,\text{dL}}\\ \amp=\frac{180}{1}\cdot\frac{1\,\text{g}}{1000}\cdot\frac{1}{10\,\text{dL}}\\ \amp=\frac{180}{1000\cdot10}\,\frac{\text{g}}{\text{dL}}\\ \amp\approx0.018\,\frac{\text{g}}{\text{dL}} \end{aligned}$$ So for example if salt is mixed into water with a concentration of 180 mg/L, the concentration can also be described as 0.018 g/dL.