1. The problem asks to decrease 400 by 10 percent. 2. To decrease a number by a percentage, we use the formula: $$\text{New value} = \text{Original value} - \left(\frac{\text{percentage}}{100} \times \text{Original value}\right)$$ 3. Here, the original value is 400 and the percentage decrease is 10%. 4. Calculate the amount to decrease: $$\frac{10}{100} \times 400 = 0.1 \times 400 = 40$$ 5. Subtract this amount from the original value: $$400 - 40 = 360$$ 6. Therefore, decreasing 400 by 10 percent results in 360.