Scientific calculations - Using photosynthesis graphs

The table shows a set of results obtained by students investigating the effect of light intensity on photosynthesis. They varied the distance of the plant from the light source. The rate of photosynthesis is the number of bubbles produced per minute.

Distance from light source in cmBubbles produced per minute
10120
1554
2030
2517
3013

If we draw a graph of this data:

A graph showing the number of bubbles produced per minute.

Note that the graph is a curve.

Question

From the graph, suggest the rate of oxygen production at a distance of 18 cm from the light source.

40 bubbles per minute.

Using the graph:

A graph showing the number of bubbles produced per minute.