The plot for January places the median age at 40, so half of the surveyed group was over 40. That would be half of 100 people, or 50 people.
The plot for July places the upper quartile at age 40, so 1/4 of the surveyed group was over 40. That would be 1/4 of 100 people, or 25 people.
There were 50 - 25 = 25 more visitors over the age of 40 at the beach in January.
