A heating coil is connected in series with a resistance r. the coil is dipped in a liquid of mass 2kg and specific heat 0.5cal `g^(-1) .^(@)C^(-1)`. A potential difference of 200V is applied and the temperature of the liquid is found to increase by `60^(@)C` in 20 minutes. if R is removed, the same rise in temperature is reached in 15minutes if R is removed, the same rise in temperature is reached in 15 minutes. find the value of R.