As part of his summer job at a restaurant, Jim learned to cook up a big pot of soup late at night, just before closing time, so that there would be plenty of soup to feed customers the next day. He also found out that, while refrigeration was essential to preserve the soup overnight, the soup was too hot to be put directly into the fridge when it was ready. The soup had just boiled at $100C,$ and the fridge was not powerful enough to accommodate a big pot of soup if it was any warmer than $20C.$ Jim discovered that by cooling the pot in a sink full of cold water, $($kept running, so that its temperature was roughly constant at $5C)$ and stirring occasionally, he could bring that temperature of the soup to $60C$ in ten minutes. How long before closing time should the soup be ready so that Jim could put it in the fridge and leave on time?