Find the smallest integer $n$ such that an $n\times n$ square can be partitioned into $40\times 40$ and $49\times 49$ squares, with both types of squares present in the partition, if
a) $40|n$; b) $49|n$; c) $n\in \mathbb N$.
To solve the problem, we need to find the smallest integer \( n \) such that an \( n \times n \) square can be partitioned into \( 40 \times 40 \) and \( 49 \times 49 \) squares, with both types of squares present in the partition. We will consider the conditions \( 40|n \), \( 49|n \), and \( n \in \mathbb{N} \).

1. **Divisibility Conditions**:
   - Since the \( n \times n \) square must be partitioned into \( 40 \times 40 \) and \( 49 \times 49 \) squares, \( n \) must be divisible by both 40 and 49.
   - The least common multiple (LCM) of 40 and 49 will give us the smallest \( n \) that satisfies both divisibility conditions.

2. **Finding the LCM**:
   - The prime factorizations of 40 and 49 are:
     \[
     40 = 2^3 \times 5
     \]
     \[
     49 = 7^2
     \]
   - The LCM is found by taking the highest power of each prime that appears in the factorizations:
     \[
     \text{LCM}(40, 49) = 2^3 \times 5 \times 7^2 = 8 \times 5 \times 49 = 8 \times 245 = 1960
     \]

3. **Verification**:
   - We need to verify that \( n = 1960 \) can indeed be partitioned into both \( 40 \times 40 \) and \( 49 \times 49 \) squares.
   - Since \( 1960 \) is divisible by both 40 and 49, we can tile the \( 1960 \times 1960 \) square with both types of squares.

4. **Conclusion**:
   - The smallest integer \( n \) such that an \( n \times n \) square can be partitioned into \( 40 \times 40 \) and \( 49 \times 49 \) squares, with both types of squares present, is \( n = 1960 \).

The final answer is \( \boxed{1960} \).