Which of the following is not a proportion 1/4 = 3/12, 4/10 = 12/20, 2/4 = 3/6, or 8/6 = 20/15

Answer 1

Cross multiply each of the listed proportions. After cross multiplying, if the LHS = RHS, then it's a proportion; otherwise it is not. I'll do the first one:

1/4 = 3/12

1·12 = 3·4

12 = 12

Yup, this one's a proportion.