Statement-1 If U is universal set and B = U - A, then n(B) = n(U) - n(A).
Statement-2 For any three arbitrary sets A, B and C, if C = A - B, then n(C ) = n(A) - n(B).

Explanation

To determine the truth of the two statements provided, we will analyze each statement step by step.

### Step 1: Analyze Statement 1
**Statement 1:** If \( U \) is the universal set and \( B = U - A \), then \( n(B) = n(U) - n(A) \).

1. **Understanding the Sets**:
- Let \( U \) be the universal set containing all possible elements.
- Let \( A \) be a subset of \( U \).
- The set \( B \) is defined as the complement of \( A \) in \( U \), which means \( B \) contains all elements in \( U \) that are not in \( A \).

2. **Counting Elements**:
- The total number of elements in the universal set \( U \) is denoted as \( n(U) \).
- The number of elements in set \( A \) is denoted as \( n(A) \).
- Since \( B \) contains all elements of \( U \) that are not in \( A \), we can express the number of elements in \( B \) as:
\[ n(B) = n(U) - n(A) \]

3. **Conclusion for Statement 1**:
- The statement is true because it correctly describes the relationship between the universal set, a subset, and its complement.

### Step 2: Analyze Statement 2
**Statement 2:** For any three arbitrary sets \( A, B, \) and \( C \), if \( C = A - B \), then \( n(C) = n(A) - n(B) \).

1. **Understanding the Sets**:
- Here, \( C \) is defined as the difference between sets \( A \) and \( B \), meaning \( C \) contains all elements that are in \( A \) but not in \( B \).

2. **Counting Elements**:
- The number of elements in set \( A \) is \( n(A) \).
- The number of elements in set \( B \) is \( n(B) \).
- However, \( n(C) \) should actually account for the elements in \( A \) that are not in \( B \). This can be expressed as:
\[ n(C) = n(A) - n(A \cap B) \]
- Therefore, the correct expression for \( n(C) \) is not simply \( n(A) - n(B) \) because \( n(B) \) does not account for the overlap between \( A \) and \( B \).

3. **Conclusion for Statement 2**:
- The statement is false because it incorrectly assumes that the number of elements in \( C \) can be calculated by simply subtracting the number of elements in \( B \) from \( A \) without considering the intersection of \( A \) and \( B \).

### Final Conclusion
- **Statement 1** is **True**.
- **Statement 2** is **False**.