6 and 15 have 3 as a common factor. Thus 6:15 is 2:5. The other ratios are also the same as given in the question. Since the numbers in the second column increase in 15s, the numbers in the first row should increase in sixes (12:30 is the same as 6:15 and 2:5). Thus the answers are:
- 6:15
- 12:30
- 18:45
- 24:60
- 30:75