Final answer:
Of the given options, the pair of numbers 7 and 9 with the least common multiple (LCM) listed as 21 is the only correct pair, as 21 is the smallest number that both 7 and 9 can divide into without leaving a remainder.
Step-by-step explanation:
The student asked which of the following pairs of numbers is correctly listed with its least common multiple (LCM):
- 5, 15; lcm: 30
- 20, 30; lcm: 60
- 24, 36; lcm: 12
- 7, 9; lcm: 21
Let's evaluate each option:
- For numbers 5 and 15, the LCM is 15, because 15 is the smallest number that both 5 and 15 can divide into without leaving a remainder.
- For numbers 20 and 30, the LCM is indeed 60. This is the smallest number that both 20 and 30 can divide into without a remainder.
- For numbers 24 and 36, the LCM is not 12; it's actually 72, which is the smallest number both can divide into evenly.
- For numbers 7 and 9, the LCM is correctly listed as 21, because it's the smallest number divisible by both without leaving a remainder.
Thus, the correct answer is:
4) 7, 9; lcm: 21.