The task is to generate the correct answer for a given math problem.

Problem: of the 100 employees at company x , 80 are full - time , and 100 have worked at company x for at least a year . there are 20 employees at company x who aren ' t full - time and haven ' t worked at company x for at least a year . how many full - time employees of company x have worked at the company for at least a year ?
20