Answer: 4 dollars for each notebook
Step-by-step explanation:
21-9= 12
12/3=4
each notebooks costs 4 dollars
The equation of c is an illustration of subject of formula
The equation of c in terms of r and t is c = r - 15/t
<h3>How to determine the equation?</h3>
The equation is given as:
t = 15/(r - c)
Cross multiply
t(r - c) = 15
Divide both sides by t
r - c = 15/t
Subtract r from both sides
-c = -r + 15/t
Multiply both sides by -1
c = r - 15/t
Take LCM

Hence, the equation of c in terms of r and t is 
Read more about subject of formula at:
brainly.com/question/10643782
The answer to the question is 1
Answer:
the relation between them are
n(U) = n(X U Y) + n(xūy).
The answer is 9(3+x)
My work is in the attached image