You can put this solution on YOUR website! The cost of one pen when pens are priced at 5 for p dollars.
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You have 2 values to use, 5 & p.
What do you think?
5*p?
5 + p?
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If you need to, sub a number for p.
eg, p = $2
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5 pens cost $2
What does 1 pen cost?