First, simplify the equation:
1. Start with the equation: 431 + (B - 29) = 678
2. Distribute and simplify: 431 + B - 29 = 678
3. Combine like terms: B + 402 = 678
4. Isolate B by subtracting 402 from both sides: B = 678 - 402
5. Calculate the right side: B = 276
Final answer: B = 276