Exercise 3: Why "Whatever's Left" Rarely Leaves Anything — Possible Solution
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This chapter's own "pay yourself first" principle addresses this
directly. Saving whatever's left at the end of the month treats
savings as the very last priority in the spending order - by the time
month-end arrives, ordinary spending (much of it not even carefully
planned) has already claimed the money, leaving little or nothing
behind regardless of how much was actually earned.

Pay yourself first reverses that order: the savings transfer happens
automatically on payday, before any other spending has a chance to
claim that money at all. This isn't just a matter of good intentions -
it's a real, structural change in timing. Money that has already moved
into a separate account the moment it's paid is no longer available to
be spent even under a moment of weak willpower later in the month,
which is a real, measurable behavioral advantage over hoping enough
will be left over after the fact.

ANSWER: The "pay yourself first" principle addresses this directly -
the real problem isn't a lack of willpower, it's that saving is being
attempted last, after spending has already claimed the money. Moving
the savings transfer to payday, automatically, removes that money from
being spendable in the first place, which is a real structural fix
rather than relying on discipline at the end of an already-busy month.

WHY THIS WORKS AS AN ANSWER
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This correctly identifies pay-yourself-first as the relevant principle
and explains why changing the TIMING of the savings action (not just
the intention) is what actually solves the "nothing left over"
problem.