Hi there! The answer is 100πcm^2
The radius of a circle can be found by dividing the diameter by 2. In other words : the radius is half of the diameter.
Therefore, the radius of a circle with a diameter of 20 cm is 10 cm.
The exact area of a circle can be found using the following formula:

In order to find our answer, we need to plug in the data from the question.

Hence, the answer is 100πcm^2.
They all end up being 1
100% = 1
4/4 = 1
1.000 = 1
2.25x15= blah blah blah, and boom there is your answer
If x is the amount that Tanya paid for each item, then Tony's items would be x-2.25. So:
3x=4(x-2.25)
3x=4x-9
x=9
x-2.25=6.75
Tanya spent $9 on each item; Tony spent $6.75 on each of his items