Answer :
[tex]\frac{24}{21}=\frac{s}{35}\\\\24\cdot35=21s\\\\21s=840\ \ \ \ \ \ \ |:21\\\\\boxed{s=40}[/tex]
[tex]\frac{24}{21}=\frac{s}{35} \\
\frac{8}{7}=\frac{s}{35} \\
35 \times \frac{8}{7}=s \\
5 \times 8=s \\
s=40
[/tex]