A heater boils 1 kg of water in time t1, and another heater boils the same water in time t2. If both are connected in series, the combination will boil the same water in time

H=V2R1×t1  or  t1=H⋅R1V2…..(1)H=V2R2×t2  or  t2=HR2V2…….(2)In series, H=V2R1+R2×tor t=HR1+R2V2………….(3)From eq.(3)  t=HR1V2+HR2V2=t1+t2Note : when the heaters ore connected in parallel.In this case, 1t1=V2HR1 and 1t2=V2HR2∴ 1t=V2H1R1+1R2=1t1+1t2or t=t1t2t1+t2