Answer:
15
Step-by-step explanation:
Since 30 and 45 are relatively small numbers, it's not a big deal to list every single one of their common factors.
Factors of 30: 1, 2, 3, 5, 6, 10, 15, 30
Factors of 45: 1, 3, 5, 9, 15, 45
Now that we have a list, it's easy to see which number is the highest common factor, which is, of course, 15.