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When μ or σ is unknown, you work backwards. Convert a probability to a z-value using the inverse standard normal, then set up an equation with the standardising formula and solve.
What you'll be able to do
Find the -value matching the given probability (inverse standard normal). Then substitute into and solve for the unknown.
Tip — Two unknowns (both μ and σ) need two probability statements → two equations.
Formula recap
Common mistakes to avoid
Key takeaways
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