Limiting Factor Approach to Questions Vs Throughput Approach to Questions

Good morning,

In preparation for my upcoming PM exam next month, I have attempted a number of Section A & Section B practice questions however I often find it extremely difficult to know whether to take a throughput approach to the question or a limiting factor approach to a question – i.e. when to only use materials as the only variable cost for a throughput question, or when to use use all variable costs for the limiting factor approach.

To demonstrate my question, I recently attempted the following exam style questions (which all had their own respective cost card for each question) which stated:

Q1. “What is the shadow price of these materials, if materials were limited to 2,000kgs per week?”

Q2. “What are the most and least profitable uses of direct labour, a scarce resource?”

Q3. “What is the production plan that would maximise profit for the next month”

Each of the above 3 questions took a limiting factor approach in the answers i.e. they all deducted all the variable costs when calculating the profitability rather than just material costs as you would do for throughput.

The following 2 questions however, took a throughput approach and correctly only deducted the material costs when calculating profitability:

Q1. “Assuming the company wants to maximise profit, what is the optimal production plan?”

Q2. “Which of the 3 processes if any, is a bottleneck activity?”

Please can you kindly advise when to use a limiting factor approach to a question, and when to take a throughput approach to a question?

Thank you so much in advance!