As you enter you see a wooden table with a pile of galleons and a metal scale in the middle of the room. One side has on it already 51 sickles and 986 knuts. How many galleons would you need to make the scale even? 
 

Did Hogwarts not teach economics!? That's OK!  If you need a little refresher on wizarding currency look no further!  https://harrypotter.fandom.com/wiki/Wizarding_currency