First subtract the 12 confiscated firecrackers from the original number: 48 firecrackers - 12 firecrackers = <<48-12=36>>36 firecrackers
Then multiply this number by 1/6 to find how many were defective: 36 firecrackers * 1/6 = <<36*1/6=6>>6 firecrackers
Then subtract the defective firecrackers to find how many were good: 36 firecrackers - 6 firecrackers = <<36-6=30>>30 firecrackers
Then divide this number in half to find how many firecrackers Jerry set off: 30 firecrackers / 2 = <<30/2=15>>15 firecrackers
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