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Three bells toll at intervals… (Class 10 Maths 2024, Q21)

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MathsCBSE Class 10 · Standard · Board exam 2024, set 30/4/1 · 2 marks · Very short answer

CBSE Class 10 Mathematics Standard 2024: question 21

Set in the CBSE Class 10 board exam of 2024, paper 30/4/1, section B, question 21, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21 · 2 marks · Very short answerCompetency-basedReal Numbers

Three bells toll at intervals of 9, 12 and 15 minutes respectively. If they start tolling together, after what time will they next toll together ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 30/4/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

The bells toll together at a time that is a multiple of every interval.

Hint 2

Find the smallest common multiple of 9, 12 and 15 using prime factorisation.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Prime factorise the intervals: , , .

    1 mark

  2. The bells next toll together after the LCM of the intervals: . So they toll together again after 180 minutes, i.e. 3 hours.

    1 mark

Answer: After 180 minutes (3 hours)

Where the marks are

  • Correct prime factorisation of 9, 12 and 15 (1 mark)
  • LCM = 180, so the bells toll together after 180 min (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Real Numbers, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 30/4/1 (Standard): every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.