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Idli-Vada: Counting Into Common Multiples
Common Multiples and Common Factors · Play-way
The Idli-Vada game makes the abstract idea of common multiples visceral before any definition is stated — let the game run energetically before drawing mathematical conclusions. A common slip is that students call 3 itself 'idli-vada'; watch for this and ask the class to check whether 3 is also a multiple of 5. The Venn diagram at the close gives a visual anchor for the intersection idea that the next units will formalise.
0–8Introduce the circle game using Activity (Idli-Vada Game), section 5.1 p.107 ("Students sit in a circle and count aloud; they replace multiples of 3...."). Run one full round with the whole class standing.
8–20Play two or three rounds, eliminating students who err. Pause after each elimination to ask the class why a particular number triggered 'idli-vada', drawing attention to numbers that are simultaneously multiples of 3 and multiples of 5.
20–32Work through Figure it Out Q1, section 5.1 p.108 ("At what number is 'idli-vada' said for the 10th time?...") and Figure it Out Q2, section 5.1 p.108 ("If the game is played from 1 to 90, find how many times....") as a class, listing multiples on the board to confirm the pattern.
32–40Draw a simple two-circle Venn diagram on the board, placing multiples of 3 and multiples of 5 in their respective regions and common multiples at the intersection. Invite students to name the smallest common multiple of 3 and 5 and record the term LCM without formal definition yet.
Homework
What if the game was played till 900? How would the answers to Q2 change? (**Figure it Out Q3, section 5.1 p.108**)
Figure it Out Q1, section 5.1 p.108 — At what number is 'idli-vada' said for the 10th time? (Find the 10th common multiple of 3 and 5.)
In the Idli-Vada game, players say 'idli' for multiples of 3 and 'vada' for multiples of 5. Which of the following numbers would make a player say 'idli-vada'?
- A 9
- B 21
- C 25
- D 30
D. 30
What each wrong option reveals
- A The student identified a multiple of 3 but did not check for simultaneous divisibility by 5.
- B The student identified a multiple of 3 but did not verify divisibility by 5.
- C The student identified a multiple of 5 but did not check for simultaneous divisibility by 3.
Also saved as 15 periods or 20 periods.
- 2Venn Diagrams and the Idli-Vada Pattern
Common Multiples and Common Factors - 3Jump Jackpot: Landing on Common Factors
Common Multiples and Common Factors - 4Reasoning with Factors: Rich Figure it Out Problems
Common Multiples and Common Factors - 5Reconstructing Venn Diagrams and Exploring LCM Boundaries
Common Multiples and Common Factors - 6Rectangles and Rows: Discovering Primes and Composites
Prime Numbers - 7The Sieve of Eratosthenes: Sifting All Primes to 100
Prime Numbers - 8Twin Primes, Digit Patterns and the 2p+1 Exploration
Prime Numbers - 9Products of Primes, Three-Prime Numbers and Digit Constraints
Prime Numbers - 10Safe Pairs: Introducing Co-prime Numbers Through the Treasure Game
Co-prime Numbers for Safekeeping Treasures - 11Co-prime Art: Thread Patterns and the Peg-Gap Rule
Co-prime Numbers for Safekeeping Treasures - 12Why Partial Factorisation Fails: Motivating Prime Factorisation
Prime Factorisation - 13Prime Factorisation Without Multiplying First
Prime Factorisation - 14Using Prime Factorisation to Test Co-primality and Divisibility
Prime Factorisation - 15Co-primality of Primes and Divisibility Reasoning
Prime Factorisation - 16Last-Digit Patterns: Divisibility by 10, 5 and 2
Divisibility Tests - 17Two-Digit and Three-Digit Endings: Rules for 4 and 8
Divisibility Tests - 18Applying All Five Tests: Palindromes, Remainders and Clever Checking
Divisibility Tests - 19Special Numbers: What Makes Each One Different?
Fun with Numbers - 20Prime Puzzle: Satisfying Row and Column Products
Fun with Numbers - 21Mixed Practice: Multiples, Factors and Prime Classification
Common Multiples and Common Factors (Revisit) - 22From Co-prime Art to Factorisation: Connecting the Strands
Co-prime Numbers for Safekeeping Treasures (Revisit) - 23Divisibility Across All Five Rules: Extended Practice
Divisibility Tests (Revisit) - 24Chapter Fluency: Mixed Problems Across All Sections
Fun with Numbers (Revisit) - 25Prime Time at Work: Four Problems, Four Methods
Synthesis
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