The Part-Whole Relationship key construct is based on the Remainder Concept. It is important to understand the problem first (conceptual understanding) by going through each problem statement, before deciding on the best method (ie the procedural step) to use to get the solution, as stated in the Introduction to ThinkingMath.
Part-Whole Relationship key construct is introduced in P4, and used further in topics of P5 and P6.
This key construct refers to problems when a proportion or fraction of a quantity is being removed, leaving a portion behind ie the remainder which is sometimes described in problems as "the remaining". In some cases, this remainder is unknown.
In Type 1, the remainder is known, represented by the fraction. The remaining parts (remainder) need not be sub-divided in the model, meaning that the model is not complex. Drawing models is thus suitable for this type of problem.
In Type 2, the problem starts with a quantity rather than a fraction being removed, and the remaining (remainder) fraction is unknown. Draw a model if one can be drawn that is easily sub-divided. Otherwise, using the Branch Method may be more efficient. Both methods are shown here for the displayed example below.

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Type 3 variation of the key construct refers to a more complex problem when both a fraction and a whole number quantity are being removed. To solve this kind of problem, reverse the change, working backwards to arrive at the initial amount before the change. Whatever has been removed must be returned first before working backwards to get the starting amount. To solve such problems, model drawing may become complicated. Thus the Branch Method becomes a very good alternative.


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Click below for other articles in our series on key constructs:
Introduction to ThinkingMath / Key constructs in Conquer Problem Sums
More Than/Less Than (Comparison, Division)
More Than/Less Than (Types 1 - 3)
Equal Stage
Repeated Identity
Equal Fractions, Internal Transfer, Guess & Check
Number x Value, External Change, Gaps & Differences