The second question uses 2×28=56 1.4×40=56, so it can form a proportion.

The third problem is that1/5× 5/8 =1/81/2×1/4 =1/8, so the proportion can be formed.

The fourth question uses1.3× 5.7 = 7.417.5× 3.1= 23.25, so it cannot form a proportion.

Ps: I hope you get the process, not the answer. Hope to adopt _