In this case the answer is very simple.
We must check the proportion.
7 : 5 = 12 : 7
7/ 5 = 1.4
12 / 7 = 1.71
7 : 5 ≠ 12 : 7
1.4 ≠ 1.71
The solution is:
7/5 and 12/7 don't form a proportion .
