Assertion: Two vectors are said to be equal if , and only if, they have the same magnitude and the same dirction.
Reason: Addition and subtraction of scalars make sense only for quantities with same units.

Explanation

To analyze the question regarding the assertion and reason provided, we will break it down step by step.

### Step 1: Understanding the Assertion
The assertion states that "Two vectors are said to be equal if, and only if, they have the same magnitude and the same direction."

**Explanation:**
- Vectors are defined by both their magnitude (length) and direction. For two vectors to be considered equal, they must have the same length and point in the same direction. This is a fundamental property of vectors in physics.

### Step 2: Evaluating the Reason
The reason states that "Addition and subtraction of scalars make sense only for quantities with the same units."

**Explanation:**
- Scalars are quantities that are fully described by a magnitude alone (e.g., mass, temperature). When adding or subtracting scalars, it is necessary that they have the same units (e.g., you cannot add kilograms to liters). This is a valid statement regarding scalars.

### Step 3: Analyzing the Relationship Between Assertion and Reason
Now we need to determine if the reason correctly explains the assertion.

- While both the assertion and reason are true statements, the reason does not provide an explanation for the assertion about vectors. The assertion is specifically about vectors, while the reason pertains to scalars. Therefore, the reason does not support or explain the assertion.

### Conclusion
- Both the assertion and reason are true, but the reason does not explain the assertion.

### Final Answer
- The assertion is true, and the reason is true but does not explain the assertion.