20% of 400 is 80. To calculate 20% of 400, we can use the formula:

• 20% * 400 = (20/100) * 400 = (20 * 400) / 100 = 8000 / 100 = 80

Alternatively, we can use the following steps:

1. Start with the given output value, which is 400.
2. Use the variable x to represent the unknown value.
3. From Step 1, we know that 400 is equivalent to 100%.
4. Similarly, we can determine that x represents 20% of the value.
5. This leads us to two simple equations: 400 = 100% (1) and x = 20% (2).
6. To solve for x, we divide equation 1 by equation 2, considering that both equations have the same unit (%). Thus, we obtain: 400 / x = 100% / 20%.
7. By taking the reciprocal of both sides of the equation obtained in Step 6, we have: x / 400 = 20 / 100 ⇒ x = 80.

Therefore, 20% of 400 is equal to 80.