A group of students investigated the effect of relative humidity on the development of the leaf cuticle. They measured the thickness of the waxy cuticle on the leaves of six different plants grown at four different levels of relative humidity. Their results are shown in the table.
| Relative humidity of environment (%) | Plant 1 cuticle thickness (µm) | Plant 2 cuticle thickness (µm) | Plant 3 cuticle thickness (µm) | Plant 4 cuticle thickness (µm) | Plant 5 cuticle thickness (µm) | Plant 6 cuticle thickness (µm) | Mean thickness of cuticle (µm) |
|---|---|---|---|---|---|---|---|
| 30 | 14.2 | 15.1 | 13.8 | 14.5 | 15.0 | 14.4 | 14.5 |
| 50 | 10.4 | 9.8 | 10.5 | 11.1 | 9.6 | 10.0 | 10.2 |
| 70 | 7.6 | 8.2 | 7.5 | 6.9 | 7.9 | 7.3 | ? |
| 90 | 4.2 | 3.8 | 4.5 | 3.9 | 4.1 | 4.1 | 4.1 |
Calculate the mean thickness of the cuticle for the plants grown at 70% relative humidity. Give your answer to one decimal place.
