My teacher solved it this way H0: μ=140 HA: μ different from 140.Significance level 0.05. Test in two sides. alpha/2=0.05/2=0.025. For this alpha and df=8, we get t(tab)=+-2.896. t cal (using the formula) is 5.
For df=9-1=8 and t=5 alpha=0.001. The difference between mean concentrations is due to chance.
But I disagree. As the t cal> t tab and alpha is 0.001<0.05, the difference is significant. Am I right?