The formula to convert Celsius to Fahrenheit is:

$$ °F = (°C × 9/5) + 32$$

Where,

F = Temperature in Fahrenheit

C = Temperature in Celsius

Substituting C = 15 in the formula, we get:

$$°F = (15 × 9/5) + 32$$

$$°F = (135/5) + 32$$

$$°F = 27 + 32$$

$$°F = 59$$

Therefore, 15 degrees Celsius is equal to 59 degrees Fahrenheit.