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VTMO 2017 — Grade 6 — semifinal Question 7

VTMO · VTMO 2017 — Grade 6 — semifinal

Question 7

Sixteen lights are off. Each operation toggles exactly three distinct lights. What minimum number of operations turns all on?
  1. $5$
  2. $6$
  3. $7$
  4. $8$
  5. $9$