IKMC 2027 - Grades 7-8 - Self-study SET 16 - Thematic Batch 3 Question 9

Each vertex and each edge of a tetrahedron is labelled with one of 1,2,3,4,5,6,7,8,9,11, each used once. The label of any edge is the sum of its endpoint labels. Edge AB is labelled 9. What is the label of CD?