The steel plate shown supports six 2-inch diameter idler rollers mounted on the plate. Two flat belts pass around the rollers, and rollers A and D are adjusted so that the tension in each belt is 10 lb. Determine (a) the resultant couple acting on the plate if a = 8 in., (b) the value of a so that the resultant couple acting on the plate is 480 lb•in. clockwise.