The formula to convert Fahrenheit to degree Celsius is:

```

°C = (°F - 32) * 5/9

```

So, to convert 78 degrees Fahrenheit to degrees Celsius:

```

°C = (78 - 32) * 5/9

= 46 * 5/9

= 230 / 9

= 25.56 °C

```

Therefore, 78 degrees Fahrenheit is equal to 25.56 degrees Celsius.