24 days
1) Consider that Bob reads at a regular pace. So, we can write out a pair of ratios. Note that if Bob reads half of a 200 pages book so he does 100 pages in 4 days
[tex]\begin{gathered} pages--------days \\ 100---------4 \\ 600---------x \end{gathered}[/tex]2) So writing out a pair of ratios we have:
[tex]\frac{100}{600}=\frac{4}{x}[/tex]Now, we can cross multiply them:
[tex]\begin{gathered} 100x=600\cdot4 \\ 100x=2400 \\ \frac{100x}{100}=\frac{2400}{100} \\ x=24 \end{gathered}[/tex]