<human>:  A pen and pencil have a total cost of $<human>: Continue.
<bot>: Define a variable ** Let x be the cost of the pencil.
How much does the pen cost? ** If the pen costs 2 times the cost of the pencil, then it costs 2x.
Write an equation ** Adding the cost of the pen and pencil we get 2x + x = 3x
How much does the pencil cost? ** Since the total cost is $6 then 3x = $6 therefore x = $6 / 3 = $2
How much does one pen cost? ** One pen is equal to 2 * x which is 2 * $2 = $4
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