To shade two-tenths (or 2/10) of a figure divided into 100 equal parts, Melanie first needs to determine how many parts that represents.
To find this, she can multiply the total number of parts (100) by two-tenths (2/10):
\[ \text{Number of parts to shade} = 100 \times \frac{2}{10} \]
Calculating that gives:
\[ 100 \times 0.2 = 20 \]
Thus, Melanie will have to shade 20 parts to show two-tenths of the figure.