[tex]\begin{gathered} \text{let us compare the ratio} \\ \text{John}\Rightarrow\text{ }\frac{15}{20}=\frac{3}{4} \\ \text{Bob}\Rightarrow\frac{27}{36}=\frac{3}{4} \\ \text{the ratio is the same, hence, they both work at the same rate} \end{gathered}[/tex]