a 2+4+6+.......................+18
1+3+5................................+17+19
1+4+7...............................+28
4+7+10...........................+19+5+9+13+...............+25
7+12+17+22+27+8+10+................................+20
2.
165:(35:x+3).19=13
3. co 3 so cs 3 chu so ma tong=3