An alloy contains zinc, copper and tin in the ratio 2:3:1 and an other contains copper, tin and lead in the ratio 5:4:3. If equal weights of both alloys are melted together to form a third alloy, then the weight of tin per kg in the new alloy will be:

$\frac{1}{4}$ kg

Let the weight of the first alloy be 12 kg,

zinc = 4kg, copper = 6kg, tin = 2kg

Similarly, if the weight of second alloy is also 12 kg 

copper = 5kg, tin = 4kg, lead = 3kg

Total weight of the third alloy = 24kg 

weight of tin = 4 + 2 = 6kg

weight per kg = $\frac{6}{24}$ = $\frac{1}{4}$ kg