Геометрическая прогрессия: b₅ = 1/162, q = 1/2.
Формула n-го члена: bₙ = b₁ * qⁿ⁻¹.
b₅ = b₁ * (1/2)⁵⁻¹ => 1/162 = b₁ * (1/2)⁴ => 1/162 = b₁ * (1/16).
b₁ = (1/162) / (1/16) = (1/162) * 16 = 16/162 = 8/81.