Answer
Interaction P-value $=0.469$. Do not reject the null hypothesis.
There is not enough evidence to conclude that there is an interaction between Mixture and Slump.
Work Step by Step
$H_0:$ there is no interaction between Mixture and Slump.
versus
$H_1:$ there is an interaction between Mixture and Slump.
In MINITAB 18, enter the data according to the table below.
Select Stat -> ANOVA -> General Linear Model -> Fit General Linear Model
In Responses enter 28-day strength (C3). In Factors enter Slump Mixture (C1 C2).
Click Model. In Factors and covariates highlight both Slump and Mixture. Click Add (to the right of Interactions through order). Slump*Mixture will appear in 'Terms in the model'. Click OK.
Click Ok.
The interaction P-value $=0.469$
