Ответ:

  1. \(57_8 = 5 \cdot 8 + 7 = 47_{10}\).

  2. \(715_8 = 7 \cdot 8^2 + 1 \cdot 8 + 5 = 448 + 8 + 5 = 461_{10}\).

  3. \(2020_8 = 2 \cdot 8^3 + 0 \cdot 8^2 + 2 \cdot 8 + 0 = 1024 + 16 = 1040_{10}\).

Ответ: 57₈ = 47₁₀; 715₈ = 461₁₀; 2020₈ = 1040₁₀.