To write 50 out of 100 as a percentage, you can use the following formula:
\[ \text{Percentage} = \left( \frac{\text{Part}}{\text{Whole}} \right) \times 100 \]
In this case, the part is 50, and the whole is 100:
\[ \text{Percentage} = \left( \frac{50}{100} \right) \times 100 = 50% \]
So, 50 out of 100 as a percentage is 50%.