3/10 = ?/20

Multiply the numerator by 2 since 10*2 = 20 --> 3*2 = 6 --> 6/20.

$$\begin{array}{rll} \frac{3}{10}&=&\frac{?}{20}\\\\ \frac{3*2}{10*2}&=&\frac{?}{20}\\\\ \frac{6}{20}&=&\frac{?}{20}\\\\ 6&=&?\\\\ ?&=&6\\\\ \end{array}$$