Let's do step by step:
First case: 3 and 6. We have to factor these numbers:


Thus the prime numbers will be 2 and 3. The Least common multiple will be:

The second case, same steps:


Thus the prime numbers will be 2,3 and 5. The Least common multiple will be:

Third case:


Thus the prime numbers will be 2,3 and 5. The Least common multiple will be:

Last case:


Thus the prime numbers will be 2 and 5. The Least common multiple will be:

Answer: The pairs that have a least common multiple of 30 are 5 and 6, and 3 and 10.