A heating coil can heat the water of a vessel from $20\,^oC$ to $60\,^oC$ in $30$ minutes. Two such heating coils are put in series and then used to heat the same amount of water through the same temperature range. The time taken now will be ............ $min$ (neglecting thermal capacity of the coils)
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A voltmeter having a resistance of $998\, ohms$ is connected to a cell of $e.m.f.$ $2\, volt$ and internal resistance $2\, ohm$. The error in the measurement of $e.m.f.$ will be