In an economy with a population of 100 million persons, 40 million hold civilian jobs and 9 million are not working but are looking for a job. the number of persons in the civilian labor force

49 million people are the civilian labor force.

Explanation:

In order to solve this you just have to remember that the civilian labor force is the number of people that are currently working, or actively looking for a job in the last 4 weeks, which are the unemployed, this does not count for the retired people, students or people that are not actively looking for a job, this means that 40 million of people working plus the 9 million of people actively looking for a job, the civilian labor force will be 49 million people.