Here is the question
aunt helen has a dress with gst inclusive price of $646.00 but she offers it to you at 0.87 of the marked price? ?
can you show me how to work it out

Your marked down price would be $562.02. I included a picture of how I got that, but I'll try to explain it as well. 0.87 in a percentage is 87% (I know this because in order to get a percentage you multiply 0.87 by 100 and get 87%). Once I did that I set up a ratio where I made 100% and $646.00 equal to each other because that is the original price. And I made 87% equal to nothing or if you want to you can put x there. I then cross multiplied 87 and 646 which gave me 56202, and then divided that number by 100 which gave me my answer of $562.02, which is 87% of your original price.  

What am i doing here. add?