Add a number to the three numbers 12, 8 and 6 to form a proportion. The number may be (), () or ()

Add a number to the three numbers 12, 8 and 6 to form a proportion. The number may be (), () or ()

4 9 or 16

2, 3, 8 and 12 can form a proportion. If the proportion is determined as 3: () = 2: (), then the proportion is ()
If a times 1 / 3 = B △ 5, then a: B = (): () if 3a-4b = 0, then a: B = (): ()
25,2,3 / 4 with a number, the composition ratio is

3:12 = 2:8
3x4=12
2x4=8

12, 13, 8, 12 can be written in proportion______ ．

Because 12 × 8 = 13 × 12, so 12:13 = 12:8 (the answer is not unique), so the answer is: 12:13 = 12:8 (the answer is not unique)