434₈ + 66₈ = (4 × 8² + 3 × 8¹ + 4 × 8⁰) + (6 × 8¹ + 6 × 8⁰) = (4 × 64 + 3 × 8 + 4 × 1) + (6 × 8 + 6 × 1) = (256 + 24 + 4) + (48 + 6) = 284 + 54 = 338₁₀
Ответ: 338