For this question, it is best to draw out the model and start with 1 unit first. So for 1 unit, 20 cent coin has 12 more than 1 unit of 50 cent coin. Once we establish this, we just draw the same unit 6 times for 20 cent coins and 5 times for 50 cent coins. Remember to replicate the 12 too as 1 unit + 12 is considered 1 unit for 20 cent coins. Once we have the model, the rest i believe it is easy for you.