Answer: 
Explanation:
The mean free path of an atom can be calculated using the following equation:
(1)
Where:
is referred to as the Universal gas constant
represents the absolute standard temperature
denotes the diameter of helium atoms
symbolizes Avogadro's number
indicates absolute standard pressure
<pFrom this, we can solve for
using
(1), aiming to determine the radius
of the helium atom:
(2)
(3)
(4)
If the radius equals half of that diameter:
(5)
Eventually:
(6)
(7)
Nonetheless, we were tasked with finding this radius in nanometers. Knowing 
:
(8)
Ultimately:
Represents the radius of the helium atom in nanometers.
Δd = 23 cm. When the eta string of the guitar has nodes at both ends, the resulting waves create a standing wave, which can be expressed with the following formulas: Fundamental: L = ½ λ, 1st harmonic: L = 2 ( λ / 2), 2nd harmonic: L = 3 ( λ / 2), Harmonic n: L = n λ / 2, where n is an integer. The rope's speed can be calculated using the formula v = λ f. This speed remains constant based on the tension and linear density of the rope. Now, let's determine the speed with the provided data: v = 0.69 × 196, yielding v = 135.24 m/s. Next, we will find the wavelengths for the two frequencies: λ₁ = v / f₁, which gives λ₁ = 135.24 / 233.08, equaling λ₁ = 0.58022 m; λ₂ = v / f₂ results in λ₂ = 135.24 / 246.94, consequently λ₂ = 0.54766 m. We'll substitute into the resonance equation Lₙ = n λ/2. At the third fret, m = 3, therefore L₃ = 3 × 0.58022 / 2, resulting in L₃ = 0.87033 m. For the fourth fret, m = 4, which gives L₄ = 4 × 0.54766 / 2, equating to L₄ = 1.09532 m. The distance between the two frets is Δd = L₄ – L₃, so Δd = 1.09532 - 0.87033, leading to Δd = 0.22499 m or 22.5 cm, rounded to 23 cm.
