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Both clocks were set right at the moment shown. Clock A gains 2 minutes an hour and clock B loses 1 minute an hour. After how many hours will A be exactly half an hour ahead of B?

Competition practiceClocks
  • A10 h
  • B15 h
  • C30 h
  • D20 h

Step-by-Step Solution

Each hour A moves 2 min ahead of true time and B falls 1 min behind, so the gap grows by 3 min an hour. A 30-minute gap needs 30 ÷ 3 = 10 h. (15 h uses only A's 2 min; 30 h uses only B's 1 min.)
Answer: A) 10 h

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