Solution:

Let \(x\) be the first part and \(y\) be the second part.

\(x + y = 20\)

The ratio is 3:7, thus \(3:7 = x:y\)

\(\frac{3}{7} = \frac{x}{y}\)

\(x = \frac{3y}{7}\)

\(\frac{3y}{7} + y = 20\)

\(\frac{10y}{7}=20\)

\(y = 14\)

\(x = 20-14 = 6\)