Anyone know the answer or the process?

Adepartment store purchases screen-printed t-shirts at a cost of $5 per shirt. they mark up the price 150% (making the selling price 250% of the store's purchase price) and put them on the sales floor. every month that a t-shirt doesn't sell, the store reduces the selling price by 25%.

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)

What is the radius of a circle with an area of 78.5 cubic inches? use 3.14 for pie