A certain substance is formed in a chemical reaction. The mass of substance formed t seconds after the start of the reaction is x grams. At any time the rate of formation of the substance is proportional to \((20 - x)\). When \(t = 0\), \(x = 0\) and \(\frac{dx}{dt} = 1\).

(i) Show that x and t satisfy the differential equation \(\frac{dx}{dt} = 0.05(20 - x)\). [2]

(ii) Find, in any form, the solution of this differential equation. [5]

(iii) Find x when \(t = 10\), giving your answer correct to 1 decimal place. [2]

(iv) State what happens to the value of x as t becomes very large. [1]