1/3, 1/4, 1/8
The answer given is not the only possible answer. You need to come up with 3 different fractions so that the LCD (least common denominator) So first, determine the prime factors of 24.
24 = 2 * 2 * 2 * 3
Now you want some denominators that will need all of those prime factors. For the above list of 3, 4, 8, only the 3 and 8 are important, since one of them uses the prime factor of 3 and the other uses 3 prime factors of 2, so the LCD for those 2 numbers is 24. The 3rd denominator could be almost anything that uses some combination of prime factors 2,2,2,3 such as 1/2, 1/4, 1/6, 1/12, etc. I just happened to pick 1/4. The only real restriction is that none of the denominators can use any extra prime factors. If they did. then the resulting LCD would not be equal to 24.
