To find out how many parts Melanie needs to shade to show two-tenths of the figure, we start with the fact that two-tenths can be expressed as a fraction:
\[ \frac{2}{10} \]
This fraction can be simplified:
\[ \frac{2}{10} = \frac{1}{5} \]
Now, since the figure is divided into 100 equal parts, we need to calculate how many of those parts represent one-fifth (or two-tenths).
To find this, we can multiply the total number of parts (100) by the fraction:
\[ 100 \times \frac{1}{5} = 20 \]
Therefore, Melanie will have to shade 20 parts to represent two-tenths of the figure.