Factors affecting photosynthesis

An experiment is set up to investigate the effect of light intensity on the rate of photosynthesis in an aquatic plant. At a distance of 24 cm from the lamp, the plant produces 10 bubbles of oxygen per minute. The lamp is then moved to 12 cm from the plant. All other conditions are kept the same.

Assuming light intensity is inversely proportional to distance squared, what is the most likely number of bubbles produced per minute at 12 cm?