Sampling Distributions · Superchallenge problem

Two surveys each record the scores of $100$ people from a population with SD $10$. By coincidence, $50$ of the people are in BOTH surveys (their scores are the same in both); the other $50$ in each survey are different, independently chosen people. What is the variance of $\bar x_1 - \bar x_2$, the difference between the two survey means?