The distribution below shows the ages (in years) of members of a trekking club:
| Age (years) | Below 20 | 20–40 | 40–60 | 60–80 | 80 and above |
| Frequency | 4 | 6 | 10 | 5 | 5 |
Assuming all classes have equal width, calculate the mean age of the members (correct to one decimal place).
(a) 49.0
(b) 42.8
(c) 50.7
(d) 54.0