Answer:
D .For every day the soap is used, the predicted mass of the soap will decrease by 1.8 grams.
Step-by-step explanation:
The given regression equation is
Predicted y = 78.5 – 1.8x where, y = mass of a bar of soap in grams and x = number of days soap has been used.
Here, slope=b=-1.8.
Slope depicts the unit increase in predicted variable due to change in independent variable.
The given slope -1.8 shows that due to change in number of days soap is used the predicted mass of the soap decreases by 1.8 units. So, we can say that "For every day the soap is used, the predicted mass of the soap will decrease by 1.8 grams".