1. The problem asks to convert 21% into millimeters. 2. Percent means per hundred, so 21% is $\frac{21}{100}$. 3. To convert a percentage to millimeters, we need a reference length because percent is a ratio, not an absolute length. 4. Without a reference length, 21% alone cannot be converted to millimeters. 5. If you provide a total length, say $L$ millimeters, then 21% of $L$ is calculated as: $$21\% \times L = \frac{21}{100} \times L = 0.21L \text{ millimeters}$$ 6. So, 21% of any length $L$ in millimeters is $0.21L$ millimeters. 7. Please provide the total length if you want a specific millimeter value.