What is the 5th triangular number? (Triangular numbers: 1, 3, 6, 10, 15, ...)
- A.10
- B.12
- C.15 ✓
- D.21
Explanation: Triangular numbers are 1, 3, 6, 10, 15, ... — the 5th one is 15.
Class 6 IMO Mathematics Olympiad practice · Algebraic Thinking · Patterns in Mathematics
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