If the samples for Ajax Extreme and Ajax Normal are 25, why the solution is with z score????? ..... According to past lessons, it should be done with Students T table. Thanks for the answer

I had the same question. Then, googled a bit and found the following answer in researchgate: " Good question. Most of the Statistical book shows when sigma is known and less than 30 sample size then z-test is appropriate. " So, I think z-test is ok here. Need to learn more though.