Sage of the Arctic
The alchemist's store appears empty, and Jeremiah walks in, following the scraping sounds of metal on metal. Covered in grime, the alchemist is hunched over a pile of rusty nails and filing away at the ends of old aluminum cans. "Jeremiah, grab a can, start filing. I have 3.8891 kilograms of rusty nail, but only 0.4464 kilograms of powdered aluminum."
"Mr. Alchemist, sir, this seems to be a lot of work. Why don't you try your experiment now, and save the leftover materials?"
"Actually... that's a really good idea. How many moles of rust will I have left if I use up all of the aluminum?