Start off by making denominator the same for blue and red. Closest would be 20, hence 4/20 and 5/20. The remainder would be 11/20, which is effectively 11 units in the model. From there, we can see that yellow makes up 3/20 of the total beads and green beads the remaining 8/20. 45 beads = 5 units, you can hence calculate green beads from there