approximately 4 million volunteers
Work Step by Step
The bar for the age group 35 to 44 represents 13 million volunteers. The bar for the age group 25 to 34 represents 9 million volunteers. The difference between the number of volunteers is: $=13-9=4$ million Thus, there are around $4$ million more volunteers aged 35 to 44 than those who are 25 to 34 years old.