AHSME 1993 Problem 18

Al and Barb start their new jobs on the same day. Al's schedule is 3 work-days followed by 1 rest-day. Barb's schedule is 7 work-days followed by 3 rest-days. On how many of their first 1000 days do both have rest-days on the same day? $\text{(A) } 48 \text{(B) } 50 \text{(C) } 72 \text{(D) } 75 \text{(E) } 100$