An investigation was carried out to study how the near point of vision changes as a person gets older.
Volunteers of different ages had their eyesight tested. A card with text was moved closer to each volunteer's eye until the text just started to become blurry. The distance between the eye and the card was then measured and recorded.
The results are shown in the table.
| Age of volunteers (years) | Near point distance (mm) | Mean near point distance (mm) | ||
|---|---|---|---|---|
| Volunteer 1 | Volunteer 2 | Volunteer 3 | ||
| 20 | 98 | 102 | 103 | 101 |
| 30 | 144 | 139 | 140 | 141 |
| 40 | 211 | 221 | 215 | ? |
| 50 | 385 | 397 | 394 | 392 |
Calculate the mean near point distance for the volunteers aged 40.
Give your answer to three significant figures.
