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SMO 2015 Junior Round 1 Question 3

SMO · SMO 2015 Junior Round 1

Question 3

How many ways are there to arrange 3 identical blue balls and 2 identical red balls in a row if the two red balls must always be next to each other?
  1. 2
  2. 4
  3. 5
  4. 10
  5. 20