Iniki has large, medium and small metal bars. The large bars each weigh 8 kg. The medium bars each weigh 6 kg. The small bars each weigh 3 kg. Iron, nickel and lead are present in the ratio 4:1:3 in each large bar, 2:1:3 in each medium bar and 1:1:1 in each small bar. If Iniki wants to melt together a combination of bars to make an alloy that contains 40 kg of iron, 20 kg of nickel and 40 kg of lead, how many small bars will she have to use?