Concept: The question tests on the basics of Probablity
Probability of selecting a red balloon = 1/6
Probability of not selecting a red balloon = 1 - 1/6 = 5/6
Probability that exactly 1 balloon is red.
Assuming the first pick is red and the other 2 are other colors, then the probability = 1/6 * 5/6 * 5/6 = 25/216
There are 3 ways of picking i.e RCC, CRC and CCR.
Therefore the required probability = 3 * 25/216 = 75/216 = 25/72