card 21 trick

[SummoningZ]

Thank you nilamo.

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Ok so now I have this import itertools

import itertools
import random
 
suits = "DCHS"
numbers = list(map(str, range(2, 11))) + list("JQKA")
full_deck = ["".join(pair) for pair in itertools.product(numbers, suits)]
random.shuffle(full_deck)
pack = full_deck[:21]

first = pack[0:7]
second = pack[7:14]
third = pack[14:21]

print("1: " + " ".join(first))
print("2: " + " ".join(second))
print("3: " + " ".join(third))

selection = int(input("> "))
if selection == 1:
  pack.extend(second)
  pack.extend(first)
  pack.extend(third)
elif selection == 2:
  pack.extend(first)
  pack.extend(second)
  pack.extend(third)
elif selection == 3:
  pack.extend(first)
  pack.extend(third)
  pack.extend(second)
  
print("You're thinking of %s" %pack[10])

How do I do the actual trick by choosing the 11th card?