1. The problem is to find the decimal fraction of $\frac{1}{400}$. 2. The formula to convert a fraction to a decimal is to divide the numerator by the denominator: $$\text{Decimal} = \frac{\text{Numerator}}{\text{Denominator}}$$ 3. Here, the numerator is 1 and the denominator is 400, so we calculate: $$\frac{1}{400}$$ 4. Perform the division: $$1 \div 400 = 0.0025$$ 5. Therefore, the decimal fraction of $\frac{1}{400}$ is $0.0025$.