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Hypothesis testing

A manufacturer purchases steel bars to make cotter pins. The claim is that the true mean tensile strength is 10,000 psi (pounds-force per square inch). The standard deviation is known to be 400 psi. This hypothesis is tested at the 5% level of significance, using a sample of size 100.
1) If the true mean strength (unknown to the manufacturer) is 9,900 psi what is the
(probability of) the type two error ?? What is the power of the test?

ii) When µ = 9900, what sample size would allow a power of 0.8?

Can anyone show me their working, especially part ii)

Thanks a LOT!

Anyone?

Anyone Please!

Really stuck, thank you