how to depict answer from this graph

Question

i am talking about the 65th question, in the below given uploaded photo, i.e. the first order decomposition curves

as per the given reaction for every mole of A3 decomposed, there is a liberation of 3/2 moles of A2 but when their concentration becomes equal, then the system is at equilibrium so. but how can i calculate the amount of reactant that has been decomposed

Answer 1

Let initial concentration be C of $\small A_3$ then after some times let it decrease by D, so $\small A_2$'s concentration increase by $3/2$ $\times$ D because of stoichiometric ratios, when equal $\small C-D=(3/2)D$ so $\small D=(2/5)C$. So concentration of $A_3$ reduced by $2/5$th or $40\%$.