The total number of flags with three horizontal strips in order, which can be formed using 2 identical red, 2 identical green, and 2 identical white strips is equal to

All strips are of different colors, then number of flags is 3! = 6. When two strips are of same colour, then number of flags is  3C1×2C1×(3!/2!)=18. Total number of flags is 6 + 18 = 24 = 4!