Which of the following Roman numerals are not valid? State the reason for your answer in each case.
VXCI

Answer

To determine whether the Roman numeral "VXCI" is valid, we will analyze each character and their arrangement according to the rules of Roman numerals.

### Step-by-Step Solution:

1. **Identify the Roman Numerals and Their Values:**
- V = 5
- X = 10
- C = 100
- I = 1

2. **Analyze the Arrangement:**
- The Roman numeral "VXCI" consists of V, X, C, and I in that order.

3. **Check the Placement Rules:**
- According to the rules of Roman numerals, a smaller numeral should not be placed before a larger numeral unless it is a specific case of subtraction (like IV for 4).
- In "VXCI", V (5) is placed before X (10). This is not allowed because V is a smaller numeral and should not precede a larger numeral.

4. **Conclusion:**
- Since V is placed before X, "VXCI" is not a valid Roman numeral.

### Final Answer:
The Roman numeral "VXCI" is not valid because V (5) is incorrectly placed before X (10), which violates the rule that a smaller numeral cannot precede a larger one unless it is a specific subtraction case.

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