Answer:6 six graders were late. 10% would be 12, and half of that is 6. You need to divide that in half because 5 is half of 10.
I think it'd be better if you gave us some sort of diagram with it?
For this case we have by definition, that the equation of a line in the slope-intersection form is given by:

Where:
m: It is the slope of the line
b: It is the cut point with the "y" axis
We have, according to the data provided, that the line is of the form:

That means that the slope is -1.
ANswer:
