A mass of 4 kg lies on a rough plane which is inclined at 30 to the horizontal. A light string has one end attached to this mass, passes up the line of greatest slope, over a smooth pulley fixed at the top oc the plane and carries a freely hanging mass of 1kg at its other end. The tension in the string is just sufficient to prevent the 4kg mass from sliding down the slope. Find the coefficient of friction between the 4kg mass and the plane.