In the C-12 and C-14 isotopes, the ratio of neutrons is x`:` y , then `x+y` is `"______"`.

Explanation

To solve the problem, we need to determine the number of neutrons in the isotopes Carbon-12 (C-12) and Carbon-14 (C-14), and then find the ratio of these neutrons.

### Step-by-Step Solution:

1. **Identify the Mass Number and Atomic Number of C-12:**
- The mass number (A) of C-12 is 12.
- The atomic number (Z) of carbon is 6.

2. **Calculate the Number of Neutrons in C-12:**
- The number of neutrons (N) can be calculated using the formula:
\[ N = A - Z \]
- For C-12:
\[ N_{C-12} = 12 - 6 = 6 \]
- Let’s denote this as \( n_1 = 6 \).

3. **Identify the Mass Number and Atomic Number of C-14:**
- The mass number (A) of C-14 is 14.
- The atomic number (Z) of carbon remains 6.

4. **Calculate the Number of Neutrons in C-14:**
- Again using the formula:
\[ N = A - Z \]
- For C-14:
\[ N_{C-14} = 14 - 6 = 8 \]
- Let’s denote this as \( n_2 = 8 \).

5. **Determine the Ratio of Neutrons:**
- The ratio of neutrons in C-12 to C-14 is:
\[ \frac{n_1}{n_2} = \frac{6}{8} = \frac{3}{4} \]
- This ratio can be expressed as \( x : y \), where \( x = 3 \) and \( y = 4 \).

6. **Calculate \( x + y \):**
- Now, we need to find \( x + y \):
\[ x + y = 3 + 4 = 7 \]

### Final Answer:
Thus, \( x + y \) is **7**.

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