Exercise 2: Adding a New Shipping Discount Rule — Possible Solution
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WHICH SUBPROBLEM NEEDS TO CHANGE
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Only CalculateShipping (Subproblem 3) needs to change. Its logic
currently reads:

    ALGORITHM CalculateShipping(discounted_subtotal)
        IF discounted_subtotal >= 75 THEN RETURN 0
        ELSE RETURN 8
        ENDIF

The new requirement adds a third case - a $5 discount off the flat $8
shipping fee specifically for orders over $200 (after discount) that
don't already qualify for free shipping. Since every order over $200
is also over $75, and the existing logic already gives those orders
free shipping (returning 0), the new rule as literally stated wouldn't
actually change anything in practice - it's worth flagging this to
whoever wrote the requirement, since "over $200 gets a $5 shipping
discount" is already fully subsumed by "over $75 gets fully free
shipping." (If the intended new threshold was actually meant to sit
BELOW the free-shipping threshold - for example, a partial discount
between $50 and $75 - CalculateShipping would still be the only
subproblem that needs updating, just with a genuinely new middle case
added between the existing two.)

WHY THE OTHER FOUR SUBPROBLEMS DON'T NEED TO BE TOUCHED
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CalculateSubtotal only sums quantity times price - it has no
awareness of discount tiers, shipping, or tax at all, so a shipping
rule change is completely outside its own responsibility.
ApplyDiscountTier only determines the percentage-off tier applied to
the subtotal itself - it doesn't know or care what happens to shipping
afterward. CalculateTax only computes tax on the discounted subtotal -
shipping cost isn't even one of its inputs. ComputeCartTotal (the
composition step) simply calls each subproblem and sums the results -
it doesn't contain any of the actual shipping RULES itself, only the
instruction to call CalculateShipping and add whatever it returns.
Since the new requirement is entirely about how shipping cost is
determined, and CalculateShipping is the one and only subproblem
whose entire responsibility is "determine shipping cost," it is the
only piece that could possibly need to change.

WHY THIS WORKS AS AN ANSWER
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The answer correctly isolates the change to the one subproblem whose
stated responsibility matches the new requirement, explicitly checks
each of the other four subproblems' own stated purpose to confirm none
of them are involved, and goes further by actually reasoning about
whether the new rule as stated changes any real behavior - rather than
mechanically assuming every new requirement necessarily changes
something.