Let y be the number of calories and x the number of minutes; therefore, according to the question, the two given ratios are
[tex]\begin{gathered} \frac{y}{x}=\frac{261}{30}=\frac{348}{40} \\ \Rightarrow\frac{y}{x}=8.7=8.7 \\ \Rightarrow y=8.7x \end{gathered}[/tex]Therefore, the constant of proportionality is 8.7, 8.7 calories per minute.