A cereal manufacturer claims to include 18.7 ounces of cereal in each box, with a standard deviation of 0.2 ounces. A random sample of 25 boxes shows an average of 18.5 ounces. Which of the following statements is correct? A. The claim is likely untrue, according to a z-value of −5.

Question

B. The claim is likely untrue, according to a z-value of −1.5.
C. The claim is likely true, according to a z-value of −5.
D. The claim is likely true, according to a z-value of −1.5.

My answers is C

Answer 1

Z = (mean1 - mean2)/standard error (SE) of difference between means

SEdiff = √(SEmean1^2 + SEmean2^2)

SEm = SD/√n

If only one SD is provided, you can use just that to determine SEdiff.

If you are using level of significance of P < .05, Z > 1.96.