Let me try to help you out with this question.
This is a concept we call “parts and units”. The name is as such because, in the above model that we draw, we include 2 types of boxes (different sizes). We call one type “unit” and another type “part”. Hence, they are also labeled “u” and “p”.
Reading the question, we understand that there are 4 units in Bag A at first, and after taking out 16, there is 8 parts left. In Bag B, there were 5 units at first, and after taking out 30, there were 9 parts left.
We write them out as 2 equations shown in the steps that are provided and we label them as ‘1’ and ‘2’. We go on to manipulate the equations ‘1’ and ‘2’ accordingly. We can see how it is solved.
We manage to find the value of ‘p’ first, which is 10.
Using the value of ‘p’ as 10, we will be able to answer the questions part a) and b).
This method used should be one of the most comprehensive model methods available. It is based-after a secondary school mathematics concept called “simultaneous equations’ but have been simplified to cater to the younger primary school students as models are used to aid the students visually.
Do PM me if you need clarification. Happy to help. (: