Siti makes a pattern using circles and triangles: circle, triangle, triangle, circle, triangle, triangle, ... This pattern continues. How many triangles are there among the first 20 shapes?
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Worked Solution
Step 1: Identify the repeating unit: circle, triangle, triangle. Each unit has 1 circle and 2 triangles. The unit length is 3.
Step 2: Divide 20 by 3: 20 ÷ 3 = 6 remainder 2. There are 6 complete units and 2 leftover shapes.
Step 3: Triangles in 6 complete units: 6 × 2 = 12 triangles.
Step 4: Identify the 2 leftover shapes using the unit pattern: shape 1 = circle, shape 2 = triangle. There is 1 more triangle in the leftover.
Step 5: Total triangles = 12 + 1 = 13.
Answer: 13.
Correct answer: 13
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