A merchant sold a pen for $6.90, thereby making a profit of 15% on her cost calculate the cost of the pen to the merchant to the nearest cent

So you know the merchant made a 15% profit on the pen, so she bought it for a cheaper price. To find the cost of the pen before you have to take the price now, $6.90 and times it by 85%. You do 85% because you subtract the 15% she saved from 100% and you get 85%. So 6.90x. 85 = 5.865 which rounds to $5.87