I have a word problem I cant figure out how they got the answer:

Three people who work full time are to work together on a project, but their total time on the project is to be equivalent to that of only one person working full time. If one of the people is budgeted for 1/2 of his time and a 2nd person for 1/3 of her time, WHAT part of the 3rd worker's time should be budgeted to this project?

the answer is 1/6 but I just cant figure how they got that?

Please help. I would really appricate it. I am sure it is simple, but I just cant see it.