If 1 pair of earrings equals 2 earrings, then Barbie bought 12*2=<<12*2=24>>24 earrings.
Half of 24 earrings is 24/2=<<24/2=12>>12 earrings
If receiving 12 earrings tripled Alissa's collection, then she now has 12*3=<<12*3=36>>36 earrings.
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