Original: 10,000
New: 8,500
You would always want to put original - new over original when it is percent of decrease. In this case, it would be 10,000 - 8,500 / 10,000. That would have 1,500 / 10,000 as an answer. Now, you would simplify, and it would give you 0.15. You switch it to percent, which is move the decimal to the right 2 times, and the answer would be 15%.
