### Leo's Homework

Every week, Leo works on the 7 problems from CMM assignments. During the 3rd week, he recorded the time for each problem, and found the following interesting facts:

- problems 1, 2, 3 together took him 6 minutes;
- problems 2, 3, 4 together took him 7 minutes;
- problems 3, 4, 5 together took him 8 minutes;
- problems 4, 5, 6 together took him 9 minutes;
- problems 5, 6, 7 together took him 10 minutes;
- problems 1, 6, 7 together took him 11 minutes;
- problems 1, 2, 7 together took him 12 minutes.