3)This was taken from an o level maths paper in 1957 ( 12 pennies = 1 shilling & 20 shillings = 1 pound) The cost of electricity used when 40 100 watt lamps are switched on for 12 hours is 4 shillings & 6 pence. Calculate the cost when 32 100 watt lamps are switched on for 20 hours.