Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
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?

### Response:
They sold 40 erasers because 20 x 2 = <<20*2=40>>40
The earned $40 from erasers because 40 x .5 = <<40*.5=20>>20
The average price of an eraser is $1 because 20 / 20 = <<20/20=1>>1
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