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Samhita, I would start by choosing 5 letters to place between x and y. I can do this in ways. I can then rearrange the order of the 5 letters in 5! ways and place them between x and y with either x at the start or y at the start. Thus I can construct this string of 7 letters in × 5! × 2 ways. Finally you can arrange the 19 remaining letters and the string of 7 letters including x and y in 20! ways. Penny
 


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