UPSC 2006: A box contains 5 sets of balls while there are 3 balls in each set. Each set of balls has one colour which is different from every other set. What is the least number of balls that must be removed from the box in order to claim with certainty that a pair of balls of the same colour has been removed?

The correct answer is option (a): 6. Watch the detailed video explanation in our Sherlocking Prelims course on UnlockIAS.

This question was asked in UPSC CSE Prelims 2006, GS Paper 1, as Question 62. It falls under Science & Technology > Quantitative Aptitude.

This question is rated "easy". It tests basic recall and most well-prepared aspirants should answer it correctly.

UPSC has asked 164 questions from Quantitative Aptitude (under Science & Technology) across multiple years. Visit the topic page to see the full list and year-wise trends.