Labour Days
\(12 \rightarrow 21\)
\(8 \quad \rightarrow \quad x\) days
\(x \times 8=12 \times 24\)
\(x=\frac{24 \times 12}{8}=36\) days

This means 8 labours will take 36 days. It is important to note that the question requested how many more days will 8 labours take to complete the same work. So, we subtract 24 days from 36 days to obtain the additional days. i.e. \(36-24=12\) days