Oi
Just calculating the differential. Step by step:

If you want to know where the tangent has no slope:
Assuming these are the angles for the inside of the triangle:
c=2
Step-by-step:
108+42+15c=180
150+15c=180
15c=30
c=2
2-0/ -31 - -32
= 2-0/ -31 + 32
= 2/1
= 2
1) 12 cookies per hour
2) train travels 80 miles per hour
Let us assume the number of student's in Zoe's class = x
Then
Number of students that have pets = 4x/5
Number of students that have rodents as pets = (1/8) * (4x/5)
= x/10
Number of students in Zoe's class
that have pets that are not rodents = (4x/5) - (x/10)
= (8x - x)/10
= 7x/10
I hope that the procedure is clear enough for you to understand and this is the answer that you were looking for.