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Q = 3.973x10⁻⁷C.

The key to solve this problem is using the equation of a parallel plate capacitor.

C = Q/Vᵃᵇ = ε₀ A/d Where C is the capacitance, Q is the charge placed on the plates, Vᵃᵇ is the potential difference between the plates, ε₀ is the constant of the permittivity of vacuum, A is the area of the plates, and d is the separation of the plates.

Clear Q from the equation Q/Vᵃᵇ = ε₀ A/d:

Q = ε₀AVᵃᵇ/d

With ε₀ = 8.85x10⁻¹²F/m, A = 5.10x10⁻³m², Vᵃᵇ = 125.00V, and d = 1.42x10⁻⁵m

Q = (8.85x10⁻¹²F/m)(5.10x10⁻³m²)(125.00V)/1.42x10⁻⁵m = 3.973x10⁻⁷C

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form