In the first alloy you don't have any lead. In one kg of the third alloy you have 0.5 kg of the first alloy and 0.5 kg of the second alloy. That means that you need to calculate how much lead you have in 0.5 kg of the second alloy. The second alloy is made out of 12 parts (5+4+3), and only three parts out of that is lead