A student investigated the rate of photosynthesis by measuring the volume of oxygen gas collected from pondweed exposed to a light source at different distances. The table below shows the results of this investigation:

\begin{array}{|c|c|c|} \hline $Distance\; of\; lamp$($cm$) & $Volume\; of\; oxygen\; collected\; in\; 15\; minutes$(${cm}^3$) & $Rate\; of\; photosynthesis$(${cm}^3$/$hour$) \ \hline 10 & 1.8 & 7.2 \ \hline 20 & 1.3 & 5.2 \ \hline 30 & 0.9 & Y \ \hline 40 & 0.5 & 2.0 \ \hline 50 & 0.2 & 0.8 \ \hline \end{array}