5 3 4 7.5 17 47 138

Options

A. 5
B. 4
C. 47
D. 17

Explanation

To identify the odd one out in the series 5, 3, 4, 7.5, 17, 47, 138, we will analyze the pattern of the series step by step.

### Step-by-Step Solution:

1. **Identify the Pattern**:
- The series seems to involve a combination of multiplication and addition. We will analyze how each number relates to the next.

2. **From 5 to 3**:
- To go from 5 to 3, we can observe that if we multiply 5 by 0.5, we get 2.5. However, to reach 3, we can consider that we are adding 0.5 to 2.5.
- So, the operation is: \( 5 \times 0.5 + 0.5 = 3 \).

3. **From 3 to 4**:
- To go from 3 to 4, we add 1 to 3.
- So, the operation is: \( 3 + 1 = 4 \).

4. **From 4 to 7.5**:
- To go from 4 to 7.5, we can multiply 4 by 1.5.
- So, the operation is: \( 4 \times 1.5 = 6 + 1.5 = 7.5 \).

5. **From 7.5 to 17**:
- To go from 7.5 to 17, we multiply 7.5 by 2 and then add 2.
- So, the operation is: \( 7.5 \times 2 + 2 = 15 + 2 = 17 \).

6. **From 17 to 47**:
- To go from 17 to 47, we multiply 17 by 2.5 and then add 2.5.
- So, the operation is: \( 17 \times 2.5 + 2.5 = 42.5 + 2.5 = 45 \).

7. **Identifying the Odd One Out**:
- According to our calculations, the expected number after 17 should be 45, but the series lists 47 instead.
- Therefore, the odd one out in the series is **47**.

### Conclusion:
The wrong number in the series is **47**; it should be **45**.