Greek Anthology Book XIV: 13. - Problem

Problem
We both of us together weigh twenty, I, Zethus, and my brother; and if you take the third part of me and the fourth part of Amphion here, you will find it makes six, and you will have found the weight of our mother.

Solution
Let $z$ be the weight in of Zethus.

Let $a$ be the weight in of Amphion.

We have:

So:


 * Zethus weighs $12$


 * Amphion weighs $8$.