Find the number of triangles in the given figure.
20232024
Find the number of triangles in the given figure.

- A.
18
- B.
20
- C.
28
- D.
34
Attempted by 2 students.
Show answer & explanation
Correct answer: C
Concept: To count triangles in a figure built from several crossing lines, work level by level: first count the simplest triangles made of a single smallest region, then the triangles formed by combining exactly two adjacent regions, then exactly three, and so on -- then add every level together, being careful not to count any triangle twice or to miss a composite shape.

Application: Label the figure as shown above, with O at the centre and E, F, G, H, I, J, K, L marking the points where the lines cross to form the smaller regions inside the outer points A, B, C, D.
Simplest (single-region) triangles: AEI, AIH, BEJ, BJF, CFK, CKG, DGL, DLH, EOJ, FOJ, FOG, LOG, HOL and HOE -- 14 triangles.
Triangles formed by combining exactly two adjacent regions: EAH, FBE, BEO, EOF, BFO, FCG, GDH, HOD, HOG and GOD -- 10 triangles.
Triangles formed by combining exactly three adjacent regions: EFH, EHG, FGH and EFG -- 4 triangles.
Total = 14 + 10 + 4 = 28 triangles.
Cross-check: As an independent consistency check, recount the triangle names actually listed at each level: level 1 lists exactly 14 distinct names, level 2 lists exactly 10 distinct names, and level 3 lists exactly 4 distinct names, with no name repeated at any level or across levels. This rules out a double-count or a dropped entry. The level-by-level method (single-region, then two-region, then three-region combinations) is itself the standard, exhaustive way to count triangles in a figure built from overlapping regions, so the confirmed recount 14 + 10 + 4 = 28 is complete and accurate.
Hence, the total number of triangles in the given figure is 28.