A flag has to be designed with 4 horizontal stripes using some or all of the colours red, green and yellow. What is the number of different ways in which this can be done so that no two adjacent stripes have the same colour?
A
12
B
18
C
24
D
36
Correct Answer: Option C
Explanation
To solve this elegantly, we will use the 'Slot Method' as seen in the 2018 'Winners Stand' question [31], [32]. We treat the 4 stripes as