To factor the expression \(20a + 16a^3\), we need to find the greatest common factor (GCF) of the terms and factor it out. The GCF of \(20a\) and \(16a^3\) is \(4a\). We then divide each term by \(4a\) and write the expression as a product of the GCF and the resulting polynomial.
Step 1: Identify the Expression
We start with the expression \(20a + 16a^3\).
Step 2: Find the Greatest Common Factor
The greatest common factor (GCF) of the coefficients \(20\) and \(16\) is \(4\). Additionally, both terms contain the variable \(a\), with the lowest power being \(a^1\). Therefore, the GCF is \(4a\).
Step 3: Factor the Expression
We factor out the GCF \(4a\) from the expression:
\[
20a + 16a^3 = 4a(5 + 4a^2)
\]
Final Answer
The factored form of the expression is \(\boxed{4a(4a^2 + 5)}\).