Explaining a statement regarding aliasing in a document
The frequencies are $\left|10\,\mathrm{Hz}+k\cdot30\,\mathrm{Hz}\right|, k\in \mathbb{Z}$ and the same for $20\,\mathrm{Hz}$.

Note that real signals always have (complex conjugate) signal components for positive and negative frequencies, so a $10\,\mathrm{Hz}$ signal has components at $\pm10\,\mathrm{Hz}$.  Consequently aliasing in a $30\,\mathrm{Hz}$ grid will also reach $\mp 20\,\mathrm{Hz}$.