Answer the following question:
Analyze the following observation table showing the variation of image-distance (v) with object-distance (u) in the case of a convex lens and answer the question that follow:
| S.no | Object distance (\(u\)) | Image distance (\(v\)) |
| 1. | \(-100\) | \(+25\) |
| 2. | \(-60\) | \(+30\) |
| 3. | \(-40\) | \(+40\) |
| 4. | \(-30\) | \(+60\) |
| 5. | \(-25\) | \(+100\) |
| 6. | \(-15\) | \(+120\) |
Write the serial number of the observation, which is not correct. -