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The average height of 50 pupils in a class is 150 cm. Five of them whose height is 146 cm, leave the class and five others whose average height is 156 cm, join. The new average height of the pupils of the class (in cm) is __________ .

Correct answer: B. 151

  • A. 149
  • B. 151
  • C. 152
  • D. 153

Explanation

The original total height was 50 pupils × 150 centimetres = 7500 centimetres. Removing five pupils removes 5 × 146 = 730 centimetres, and the five new pupils add 5 × 156 = 780 centimetres, so the new total is 7550 centimetres. The new average is 7550 centimetres / 50 pupils = 151 centimetres. The likely mistake is averaging 146 centimetres and 156 centimetres directly or forgetting that the class size remains 50 pupils.

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