An investigation was carried out to study how age affects the near point of vision (the closest distance at which the eye can focus clearly).
Volunteers of different ages were tested. Each volunteer moved a card with small printed text closer to their eyes until the letters first began to blur. This distance was measured and recorded.
The results are shown in Table 1.
Table 1
| Age of volunteers (years) | Distance for clear vision (mm) | Mean distance (mm) | ||
|---|---|---|---|---|
| Person 1 | Person 2 | Person 3 | ||
| 20 | 115 | 112 | 118 | 115 |
| 30 | 145 | 149 | 144 | 146 |
| 40 | 211 | 218 | 214 | X |
| 50 | 345 | 352 | 347 | 348 |
Calculate the mean distance X for the volunteers aged 40.
Give your answer to three significant figures.
