Answer: The cup cost $5.
Explanation:
Let c be the cost of a cup; and let t be the price of the cost of the teapot.
By looking the question, we can list the following system equation.
c + t = 110 1st equation
t = c + 100 2nd equation
We plug in the equation c + 100 into the first equation as t, we get
c + c + 100 = 110
2c + 100 = 110
- 100 -100
2c = 10
/2 /2
c = 5