The total number of boron - oxygen bonds in borax is 'x' and boron - oxygen - boron bonds are 'y'. Then the value of `x-y` is :

Answer

To solve the problem, we need to determine the values of \( x \) (the total number of boron-oxygen bonds in borax) and \( y \) (the number of boron-oxygen-boron bonds in borax) and then find \( x - y \).

### Step-by-Step Solution:

1. **Identify the Chemical Formula of Borax**:
Borax is chemically known as sodium tetraborate decahydrate, with the formula \( Na_2B_4O_7 \cdot 10H_2O \). However, for this problem, we will focus only on the boron and oxygen components.

2. **Draw the Structure of Borax**:
The structure of borax consists of boron atoms connected to oxygen atoms. Each boron atom can form bonds with oxygen atoms, and some of these oxygen atoms are shared between boron atoms.

3. **Count the Total Boron-Oxygen Bonds (x)**:
- In the structure, there are 4 boron atoms and 7 oxygen atoms.
- Each boron atom typically forms 4 bonds, and since some of these bonds are shared with other boron atoms, we need to count the unique boron-oxygen bonds.
- After analyzing the structure, we find that there are a total of 12 boron-oxygen bonds.

Therefore, \( x = 12 \).

4. **Count the Boron-Oxygen-Boron Bonds (y)**:
- A boron-oxygen-boron bond occurs when an oxygen atom is shared between two boron atoms.
- In the structure of borax, we can identify these connections. After counting, we find that there are 5 boron-oxygen-boron bonds.

Therefore, \( y = 5 \).

5. **Calculate \( x - y \)**:
- Now that we have both values, we can find \( x - y \):
\[ x - y = 12 - 5 = 7 \]

### Final Answer:
The value of \( x - y \) is \( 7 \).