Product of Composite Number with Number is Solid Number

Theorem
Let $a, b \in \Z$ be positive integers.

Let $a$ be a composite number.

Then $a b$ is a solid number.

Proof
By definition of composite number:
 * $\exists p, q \in \Z_{>1}: a = p q$

Then:
 * $a b = p q b$

Hence the result by definition of solid number.