X=D(p)=400-p

In the expression X = D(p) = 400 - p, X represents a dependent variable, and p represents an independent variable. The function D(p) is defined as the difference between 400 and p.

To find the value of X for a given value of p, you can substitute the value of p into the equation and evaluate the expression.

For example, if you want to find the value of X when p = 300, you would substitute p = 300 into the equation:

X = D(300) = 400 - 300
X = 100

Therefore, when p = 300, X is equal to 100.

You can also use this equation to determine the value of p when X is known. In this case, you would rearrange the equation to solve for p:

X = D(p)
X = 400 - p
p = 400 - X

For example, if you know that X = 150, you can substitute X = 150 into the equation:

p = 400 - 150
p = 250

Therefore, when X = 150, p is equal to 250.

The equation X = D(p) = 400 - p represents a linear function, where X decreases by 1 for every 1 increase in p.