And to do it the other way around: 72 Fahrenheit is 40/2, which is 20 Celsius. You can even use 30 instead of 32, if that lets you do the math quicker—the answer will still be close enough.

Consider that 3 degrees Fahrenheit is the difference between a raging fever and a healthy toddler. Between a hockey rink and a swimming pool. Between food going bad or staying at a safe temperature.