The way one can work out what fraction the shorter rod is of the longer one is by essentially finding an LCM and simplifying the ratio into an 1:n form.
Firstly, to find the lowest common multiple between the 2, you will need some sort of unit or measure to calculate with. I used the interactivity's grid squares and measured each square as 1 unit. Using this, you can find an LCM between the lengths of the 2 rods. Using this, I then formed a ratio. For example, if we were to use the yellow and dark green rods, they both have a length of 5 and 6 units respectively and have an LCM of 30. This signifies that if the 2 types of rods were placed consecutively under they both met at a column, there would be a total length of 30.
This means that there are (30/5) 6 yellow rods and (30/6) 5 dark green rods. This allows us to form a ratio that there are 5 dark green rods to every 6 yellow rods. If put in the form of 1:n, there would be 1 green rod to every yellow rod - 1:1.2
If you didn't have the grid squares, you would just consecutively line up the rods until they meet, count how many of each there are, form a ratio and use the same steps.