15% would be 8.4585
hope this helps!!
Here are some simple equations that can be a life saver with this stuff :
vol of a sphere =

Circumference =

sorry this might not be very helpful. I haven't don't this since last year. You basically have to work backwards. so plug in 972 for V and then do (3/4)×3.14.
then take the answer u got for ^^ and divide it by 2? not sure.
Answer:
Find annual profit: $75,000/6 = $12,500
ROI = Annual Profit/ initial investment
ROI = $12,500/$15,000 or 83.3%
hope that help