CBSE Math Revision Start revising
Themed maths

Maths about me: a first-lesson icebreaker for CBSE Class 9 and 10

A first lesson of the session that gets everyone doing maths straight away: prime factorisation, HCF and LCM built from a birthday, ‘Who am I?’ riddles in pairs and the four 4s challenge. Everything is on paper and nothing personal is collected.

Level
CBSE Class 9 and 10
Time
40 minutes, plus a 10-minute extension
Topics
Real numbers: prime factorisation, HCF and LCM; Number system: rational numbers; Order of operations
Equipment
No calculator. Every answer is a whole number or an exact fraction.

Download student sheet (PDF)Answers (PDF)

Suggested timings

PartTimeWhat
Starter5 minQuick questions on the board
Main: task A12 minMaths about me
Main: task B10 min‘Who am I?’ in pairs
Main: task C13 minFour 4s
Extension10 minFast finishers or homework

Starter (5 minutes)

‘Who am I?’ riddles on the board as students arrive.

  1. I am a two-digit prime. My digits add up to 10 and my tens digit is bigger than my units digit. Who am I?
  2. I am a square number between 50 and 100, and I am 1 more than a multiple of 8. Who am I?
  3. I am the HCF of 96 and 404. Who am I?
  4. I am the smallest number that 8, 9 and 12 all divide exactly. Who am I?

Main activity (35 minutes)

Task A: Maths about me (12 min)

Work through Ravi’s made-up numbers first. Then do the same with your own birthday and house number on paper.

  1. Ravi’s birthday is the 18th, and his house number is 120. Write 18 and 120 as products of primes.
  2. Find the HCF and the LCM of 18 and 120. Check that HCF × LCM = 18 × 120.
  3. Ravi was born in month 8, so he writes 1808. Write 1808 as a product of primes, and find the HCF of 1808 and 18.

Task B: ‘Who am I?’ in pairs (10 min)

Solve these with a partner. Then each write a riddle of your own with exactly one answer and swap.

  1. I am between 100 and 200. I am a multiple of 7 and of 11, and my digits add up to 10. Who am I?
  2. I am the smallest number bigger than 1 that leaves remainder 1 when divided by 2, 3, 4, 5 or 6. Who am I?
  3. I am a rational number between 1/3 and 1/2. Written in lowest terms, my denominator is 12. Who am I?

Task C: Four 4s (13 min)

Use exactly four 4s with +, −, ×, ÷ and brackets to make each target. You may also join two 4s to make 44. Other answers are possible.

  1. Make 2.
  2. Make 7.
  3. Make 9.
  4. Make 20.

Extension (10 minutes)

For fast finishers.

  1. Make 15 and 17 with four 4s.
  2. Find every two-digit number that is 4 times the sum of its digits.

For teachers

Teacher notes and full worked answers
  • Nothing personal is collected: students work on paper, and nothing is typed into the site or stored. If a student would rather not use a real birthday, any date works.
  • Pairs for task B: each student writes one riddle with exactly one answer and swaps with a partner, who checks that only one number fits.
  • Mindset prompts for the first lesson (for you to read out or put on the board; nothing is collected or stored): ‘A time maths felt hard, and what helped’; ‘One topic from last year I could teach a friend’; ‘What I do when I am stuck’; ‘How I want this class to feel’.
  • Task A practises the Fundamental Theorem of Arithmetic from the Real numbers chapter: every whole number above 1 has exactly one prime factorisation.

Starter

  1. 73
    • Two-digit primes with digit sum 10: 19, 37, 73.
    • Only 73 has the bigger tens digit.
  2. 81
    • The squares are 64 and 81.
    • 81 = 8 × 10 + 1
  3. 4
    • 96 = 25 × 3 and 404 = 22 × 101
    • HCF = 22 = 4
  4. 72
    • 8 = 23, 9 = 32, 12 = 22 × 3
    • LCM = 23 × 32 = 72

Task A: Maths about me

  1. 18 = 2 × 32 and 120 = 23 × 3 × 5
    • 18 = 2 × 9 = 2 × 3 × 3
    • 120 = 2 × 60 = 2 × 2 × 30 = 2 × 2 × 2 × 15 = 23 × 3 × 5
  2. HCF = 6, LCM = 360; 6 × 360 = 2160 = 18 × 120
    • HCF = 2 × 3 = 6 (lowest powers of the common primes)
    • LCM = 23 × 32 × 5 = 360 (highest powers)
    • 6 × 360 = 2160 and 18 × 120 = 2160
  3. 1808 = 24 × 113; HCF = 2
    • 1808 ÷ 2 = 904, ÷ 2 = 452, ÷ 2 = 226, ÷ 2 = 113, and 113 is prime.
    • 18 = 2 × 32: the only common prime is 2, once. HCF = 2.

Task B: ‘Who am I?’ in pairs

  1. 154
    • Multiples of 77 between 100 and 200: 154 only.
    • 1 + 5 + 4 = 10
  2. 61
    • LCM(2, 3, 4, 5, 6) = 60
    • 60 + 1 = 61
  3. 5/12
    • 1/3 = 4/12 and 1/2 = 6/12.
    • Between them: 5/12, already in lowest terms.

Task C: Four 4s

  1. 4/4 + 4/4 = 2
    • 1 + 1 = 2
  2. 44/4 − 4 = 7
    • 44 ÷ 4 = 11, and 11 − 4 = 7
  3. 4 + 4 + 4/4 = 9
    • 8 + 1 = 9
  4. (4 + 4/4) × 4 = 20
    • 4 + 1 = 5, and 5 × 4 = 20

Extension

  1. 4 × 4 − 4/4 = 15 and 4 × 4 + 4/4 = 17
    • 16 − 1 = 15 and 16 + 1 = 17
  2. 12, 24, 36 and 48
    • 10a + b = 4(a + b) gives b = 2a.
    • a = 1, 2, 3, 4 give 12, 24, 36, 48.

The same answers are in the answers PDF. Every answer was recomputed by computer before publishing.

Practise the topics

Estimation station and class survey maths · Prior-knowledge relay and maths bingo maths · Christmas escape room maths · All themed maths

More for lessons: Weekly starters · Skill Builders · Worksheet builder · Olympiad and competition maths · National Mathematics Day puzzles