Say the listed price is "p". The customer gets a 20% discount, so each pen costs 0.8p. But the customer is also getting 20% more pens than he is paying for (he gets 2 free when he buys 10). So he is really paying 0.8p for 1.2 pens (20% more than 1), and the cost per pen to him is 0.8p/1.2 = 2p/3.
Now we know this is still 20% greater than the cost of each pen to the seller. Let's call that cost price "c". Then we know the discounted price is 1.2 times c, or 6/5 of c, so we have
2p/3 = 6c/5
p = (3/2)(6c/5)
p = 18c/10 = 1.8c
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