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Previous Year Question (PYQs)
2
In how many ways can the following diagram be colored, subject to: (i) Each of the smaller triangles is to be painted with one of three colors — red, blue, or green. (ii) No two adjacent regions have the same color.
Solution
The figure has 4 small triangles (1 top, 3 at the base).
Let top be colored in 3 ways.
The 3 base triangles are adjacent, so they must all be different and also different from the top if adjacent.
⇒ Choose color for top: 3 ways
⇒ Choose 3 distinct colors for bottom row (arranged 3! = 6 ways).
⇒ But bottom middle and top are adjacent → exclude same color combinations.
Valid colorings = $3 \times (3! - 3! / 3) = 3 \times 8 = 24.$
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