They sold 40 erasers because 20 x 2 = <<20*2=40>>40
Therefore 20(Pencil Price) + (1/2)(40 x Pencil Price) = 80
Therefore 40(Pencil Price) = 80
Therefore (Pencil Price) equals 2 because 80 / 40 = <<80/40=2>>2
An eraser costs $1 because 2 / 2 = <<2/2=1>>1
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