The scales have two major differences between them - their degrees aren't the same size, and they each have different starting points. An increase of one degree Celsius is a bigger change in heat than one degree Fahrenheit (1.8 times as much), so there's a specific ratio between the two. And the Celsius scale sets its zero point at the freezing point of water, while the Fahrenheit scale sets its zero at a particular chemical reaction of ice and ammonium chloride (called a frigorific mixture), because it was consistent to reproduce and always comes to the same temperature.

So the conversion from Celsius to Fahrenheit, you multiply by 1.8 (the ratio between their degrees) and add 32. And going from F to C, you do that backwards - subtract 32, then divide by 1.8. So 50°C using that formula gets us to 122°F.