I'm not a Geometry Teacher, but I am good at it to help you with your question. :)
Since you're adding pieces of olives around the crust of the pizza, you will need the radius of the pizza to get the area of the pizza all the way around.
Diameter 20 inches.
20 / 2 = 10
Radius is 10 inches since you divide the diameter by 2 to get the radius of a circular object.
Take 2TT and multiply it by 10 and you will get 62.8. Round it to 63.
Take the 63 and divide it by 8 since they're asking for 8 slices of pizza with olives on the crust, and you will get 7.8. Approximately 8 after rounding 7.8.
Equation example:
2TT * 10 = 62.8 = 63
63/8 = 7.8 = 8.
I hope this helps you with your question.
If you need help with anything else or more advanced understanding for something, let me know.
Have a good day.