Challenging Permutation question 1

Q: There are five buildings. There are 4 different colours of paint. Each building must be painted with only 1 colour. Find the number of ways to paint all the 5 buildings if all 4 colours must be used.

Solution: 5 Choose 2 to have the same colour, then multiply by 4! for the colour permutation. Ans = 240

Principle used: Grouping principle.

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