The ratio of the number of red pens to the number of black pens in a bookstore was $$3 : 7$$. An equal number of red pens and black pens were sold. In the end, the ratio of the number of red pens left to the number of black pens left was $$5 : 13$$. There were $$20$$ red pens in the end. How many red pens were sold? 
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