630 : (30 - у) - 45 = 25.

To solve the equation \( 630 : (30 - y) - 45 = 25 \), let's follow these steps:

1. First, isolate the fraction term: \[ 630 : (30 - y) = 25 + 45 \] \[ 630 : (30 - y) = 70 \]

2. Now, rewrite the division as multiplication: \[ \frac{630}{30 - y} = 70 \]

3. Cross-multiply to eliminate the fraction: \[ 630 = 70(30 - y) \]

4. Distribute the 70: \[ 630 = 2100 - 70y \]

5. Now, isolate the term involving \( y \): \[ 70y = 2100 - 630 \] \[ 70y = 1470 \]

6. Finally, solve for \( y \): \[ y = \frac{1470}{70} = 21 \]

So, the value of \( y \) is \( 21 \).