$(NET 3)$ Let $x_1, x_2, x_3, x_4,$ and $x_5$ be positive integers satisfying
\[x_1 +x_2 +x_3 +x_4 +x_5 = 1000,\]
\[x_1 -x_2 +x_3 -x_4 +x_5 > 0,\]
\[x_1 +x_2 -x_3 +x_4 -x_5 > 0,\]
\[-x_1 +x_2 +x_3 -x_4 +x_5 > 0,\]
\[x_1 -x_2 +x_3 +x_4 -x_5 > 0,\]
\[-x_1 +x_2 -x_3 +x_4 +x_5 > 0\]
$(a)$ Find the maximum of $(x_1 + x_3)^{x_2+x_4}$
$(b)$ In how many different ways can we choose $x_1, . . . , x_5$ to obtain the desired maximum?
1. **Rewriting the problem in terms of new variables:**
   Let \( x_1 = a \), \( x_2 = b \), \( x_3 = c \), \( x_4 = d \), and \( x_5 = e \). The given conditions become:
   \[
   a + b + c + d + e = 1000,
   \]
   \[
   a - b + c - d + e > 0,
   \]
   \[
   a + b - c + d - e > 0,
   \]
   \[
   -a + b + c - d + e > 0,
   \]
   \[
   a - b + c + d - e > 0,
   \]
   \[
   -a + b - c + d + e > 0.
   \]

2. **Analyzing the inequalities:**
   We can rewrite the inequalities as:
   \[
   a - b + c - d + e > 0,
   \]
   \[
   a + b - c + d - e > 0,
   \]
   \[
   -a + b + c - d + e > 0,
   \]
   \[
   a - b + c + d - e > 0,
   \]
   \[
   -a + b - c + d + e > 0.
   \]

3. **Summing the inequalities:**
   Adding the first and third inequalities:
   \[
   (a - b + c - d + e) + (-a + b + c - d + e) > 0,
   \]
   \[
   2c - 2d + 2e > 0,
   \]
   \[
   c - d + e > 0.
   \]

   Adding the second and fifth inequalities:
   \[
   (a + b - c + d - e) + (-a + b - c + d + e) > 0,
   \]
   \[
   2b - 2c + 2d > 0,
   \]
   \[
   b - c + d > 0.
   \]

4. **Finding the maximum of \((a + c)^{b + d}\):**
   We need to maximize \((a + c)^{b + d}\) under the constraint \(a + b + c + d + e = 1000\). From the inequalities, we have:
   \[
   b + d < a + c + e,
   \]
   \[
   2(b + d) < 1000,
   \]
   \[
   b + d \leq 499.
   \]

   Similarly,
   \[
   a + c \leq 499.
   \]

   The maximum of \((a + c)^{b + d}\) is attained when \(a + c = b + d = 499\) and \(e = 2\).

5. **Checking the conditions:**
   For \(a + c = 499\) and \(b + d = 499\), we need to ensure the inequalities hold:
   \[
   c + e < a + b + d,
   \]
   \[
   c < a + 497,
   \]
   \[
   b < d + 497.
   \]

   This implies \(a > 1\) and \(d > 1\).

6. **Counting the number of solutions:**
   The number of ways to choose \(a\) and \(d\) such that \(a, d \in \{2, \ldots, 498\}\) is \(497^2\).

The final answer is \( \boxed{ (a + c)^{b + d} = 499^{499} } \) and the number of ways to choose \(x_1, \ldots, x_5\) to obtain the desired maximum is \(497^2\).