<human>:  A store sells pencils and erasers. It has a rule that for every pencil you buy, you must buy 2 erasers which cost 1/2 the price of the pencils. If they sold 20 pencils and earned $80, how much do erasers cost?
<bot>: How many erasers did they sell? ** They sold 40 erasers because 20 x 2 = <<20*2=40>>40
How much did they earn? ** Therefore 20(Pencil Price) + (1/2)(40 x Pencil Price) = 80
How much did they earn? ** Therefore 40(Pencil Price) = 80
How much does each pencil cost? ** Therefore (Pencil Price) equals 2 because 80 / 40 = <<80/40=2>>2
How much does each eraser cost? ** An eraser costs $1 because 2 / 2 = <<2/2=1>>1
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