For normal adjustment, the tube length L = f₀ + fₑ = 105, and the magnifying power M = f₀/fₑ = 20. From the second equation, f₀ = 20fₑ. Substituting this into the first equation: 20fₑ + fₑ = 105 => 21fₑ = 105 => fₑ = 5 cm. Then, f₀ = 20 * 5 = 100 cm.