Tax Rate Applications On Mix Items

I buy a metal scrap which contains a mixture of two or more metals coming under the slab of 3% and 18% both respectively.
it contains about 80% of metal with 18% tax rate and about 20% of metal with 3% tax rate.
Which tax rate should i apply in buying that stock which is mixed in indefinite proportion i.e., not fixed.
The seller applies 3% tax rate on the whole of metal scrap which is questionable to gst officers.
Kindly clarify if there is any act or rule suggesting the application of tax in mix lot of items with different tax rates.