Shirts, numbered consecutively from 1 to 20 are worn by 20 members of the bowling league.

Shirts, numbered consecutively from 1 to 20 are worn by 20 members of the bowling league. If any three of these members are chosen to be a team, the sum of their shirt humbers will be used as a code for this team. Show that if any eight of the 20 are preselected, then from these eight one may form at least two different teams having the same code number.