Two ratios are said to be in proportion if they are equivalent or equal.
Consider 3:5 :: 12:20
$12:20=\frac{12}{20}=\frac{3}{5}=3:5$
Hence, they are in proportion
Consider 1:2::3:6
$3:6=\frac{3}{6}=\frac{1}{2}=1:2$
Hence, they are in proportion.
Note that 2: 3 and 4: 9 are not in proportion as the product of extremes (2 $\times$ 9 = 18) is not equal to product of means (3  $\times$ 4 = 12).
Similarly, 1:2 and 4:5 are not in proportion as the product of extremes (1 $\times$ 5 = 5) is not equal to product of means (2  $\times$ 4 = 8).