A student investigated the effect of light intensity on the rate of photosynthesis in pondweed. They placed the light source at different distances from the pondweed and counted the number of oxygen bubbles released per minute. Their results are shown in Table 1.
| Distance of light source (cm) | Number of bubbles in 1 minute | |||
|---|---|---|---|---|
| Test 1 | Test 2 | Test 3 | Mean | |
| 15 | 35 | 37 | 36 | 36 |
| 30 | 18 | 15 | 18 | [X] |
| 45 | 7 | 8 | 9 | 8 |
Calculate the mean number of bubbles per minute (value [X]) when the light source was at a distance of 30 cm.
