4. A cable immersed in a testing tank is charged to a voltage of 200 V, an electrostatic voltmeter being connected betwen core and tank. After one minute's electrification, the cable and voltmeter are isolated and is found that in 20 s the voltage falls to 150 V. The test is repeated with a resistance of 20 MQ between core and tank and the voltage is found to fall to 100 V in the same time. Calculate the insulation resistance of the cable, that of the voltmeter being given as 50 MQ.