Given:
For 12 hours of work, david made $168.
To find the required time to make $238.
Since, it is in direct proportion.
We can write it as,
[tex]\begin{gathered} 12\colon168\Colon x\colon238 \\ x\times168=12\times238 \\ x=\frac{12\times238}{168} \\ x=17 \end{gathered}[/tex]Hence, the required time to make $238 is 17 hours.