Find all polynomials $ P(x)$ with real coefficients such that for every positive integer $ n$ there exists a rational $ r$ with $ P(r)=n$.
1. **Assume \( P(x) \) is a polynomial with real coefficients such that for every positive integer \( n \), there exists a rational \( r \) with \( P(r) = n \).**

2. **First, note that \( P(x) \) cannot be a constant polynomial.** 
   - If \( P(x) \) were constant, say \( P(x) = c \), then \( c \) would have to be equal to every positive integer \( n \), which is impossible. Therefore, \( \deg P \geq 1 \).

3. **Let \( d = \deg P \geq 1 \).** 
   - For each \( n \in \mathbb{N} \), there exists a rational number \( r_n \) such that \( P(r_n) = n \).

4. **Consider the values \( P(r_1) = 1, P(r_2) = 2, \ldots, P(r_{d+1}) = d+1 \).** 
   - Using Lagrange interpolation, we can construct a polynomial \( Q(x) \in \mathbb{Q}[x] \) that passes through these points. Since \( P(x) \) and \( Q(x) \) agree on \( d+1 \) points and both have degree \( d \), they must be identical. Hence, \( P(x) \in \mathbb{Q}[x] \).

5. **Since \( P(x) \in \mathbb{Q}[x] \), we can write \( P(x) \) as a polynomial with rational coefficients.** 
   - Let \( t \) be the least common multiple of the denominators of the coefficients of \( P(x) \). Then \( tP(x) \in \mathbb{Z}[x] \) with leading coefficient \( m \in \mathbb{Z} \setminus \{0\} \).

6. **For each \( n \in \mathbb{N} \), \( tP(x) - tn \in \mathbb{Z}[x] \) has a rational root \( r_n \).** 
   - The leading coefficient of \( tP(x) - tn \) is \( m \). Therefore, the denominator of \( r_n \) must divide \( m \). This implies \( r_n \in \frac{1}{m} \mathbb{Z} \) for all \( n \in \mathbb{N} \).

7. **Assume \( d = \deg P \geq 2 \).** 
   - As \( |x| \to \infty \), \( \left| \frac{P(x)}{x} \right| \to \infty \). Therefore, there exists \( N \in \mathbb{N} \) such that \( \left| \frac{P(x)}{x} \right| > 2|m| \) for all \( |x| \geq N \).

8. **Consider the set \( \frac{1}{m} \mathbb{Z} \cap (-N, N) \).** 
   - This set contains exactly \( 2|m|N - 1 \) elements. Among the \( 2|m|N \) different numbers \( r_1, r_2, \ldots, r_{2|m|N} \in \frac{1}{m} \mathbb{Z} \), there must be some \( k \) such that \( |r_k| \geq N \).

9. **For such \( r_k \), we have \( 2|m| < \left| \frac{P(r_k)}{r_k} \right| = \frac{k}{|r_k|} \leq \frac{2|m|N}{N} = 2|m| \).** 
   - This is a contradiction.

10. **Therefore, \( \deg P = 1 \).** 
    - Hence, \( P(x) \) must be a linear polynomial with rational coefficients, which is indeed a solution to the problem.

The final answer is \( \boxed{ P(x) \in \mathbb{Q}[x] } \) with \( \deg P = 1 \).