We need to solve the equation $40 - x + x + 312 - x = 400$ for $x$.

1. Problem Description

We need to solve the equation $40 - x + x + 312 - x = 400$ for $x$.

2. Solution Steps

First, we simplify the left side of the equation:

$40 - x + x + 312 - x = 400$

Combine like terms:

$(40 + 312) + (-x + x - x) = 400$

$352 - x = 400$

Next, we isolate $x$ by subtracting 352 from both sides of the equation:

$352 - x - 352 = 400 - 352$

$-x = 48$

Finally, we multiply both sides of the equation by -1 to solve for $x$:

$(-1)(-x) = (-1)(48)$

$x = -48$

3. Final Answer

$x = -48$