To know the ratio, you need first to convert the numbers into fractions.
Cancelling out the unit, 0.3/0.45 = 0.02/0.03 or 0.02 is to 0.03 or 0.02:0.03.
We could also remove the decimals by moving both sides by two places to the right making 0.02 as 2 and 0.03 as 3 making the ratio as 2:3.