Mixture of NaOH and Na2CO3 titrated with HCl. Two endpoints: (1) Phenolphthalein endpoint (pH ~8.3): NaOH + HCl -> NaCl + H2O (all NaOH neutralised) AND Na2CO3 + HCl -> NaHCO3 + NaCl (half of Na2CO3 reacted). Volume = V1. (2) Methyl orange endpoint (pH ~4): NaHCO3 + HCl -> NaCl + H2O + CO2 (remaining half of Na2CO3 reacted). Additional volume = V2. From V2: meq Na2CO3/2 = N x V2. So meq Na2CO3 = 2NV2. From V1: meq NaOH = N(V1 - V2). If V1 < V2: impossible for NaOH + Na2CO3 mixture; indicates Na2CO3 + NaHCO3 mixture instead.