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Lesson 4 of 15

Reading it off the two prime lists

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7 min

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Take 84 and 96. Listing every factor takes too long.

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12 = 2 × 2 × 3.
Step 1 of 5
  1. 12 = 2 × 2 × 3.
  2. 18 = 2 × 3 × 3.
  3. Pair a 2 with a 2, a 3 with a 3.
  4. One 2 and one 3 stay unpaired.
  5. Paired pieces: 2 × 3 = 6.

Pair the lists off

Write both prime lists. Pair each prime with the same prime in the other list. Each pair is a prime that both numbers have. Multiply the pairs. That is the HCF. A piece with no partner is left out.

Now 84 and 96:

  • 84 = 2 × 2 × 3 × 7
  • 96 = 2 × 2 × 2 × 2 × 2 × 3
  • Pairs: 2, 2 and 3. HCF = 2 × 2 × 3 = 4 × 3 = 12.
  • Left out: the 7, and three of the 2s in 96.

Try the lab to finish this lesson.