I need help with 1-4

Ifurniture stores having a weekend sale and is offering 20% discount on patio chairs and table the sales tax on furniture is 6.25 using function composition how can you represent the total amount a that you would need to pay for this furniture that cost x dollars

Aright cylinder has a diameter of 8 m and a height of 6m. what is the volume of the cylinder

What is the solution for 6 - 7/8? explain the steps.