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Density of rational numbers · 2 marks

Find 5 rational numbers between 25 and 35.

Answer: Multiply top and bottom by 6: 1230 and 1830. Five numbers between: 1330, 715, 12, 815, 1730.

Step-by-step solution

Idea: The numerators 2 and 3 have no whole number between them. Multiply top and bottom by 5 + 1 = 6 to make room for exactly 5.

  1. 25 = 2 × 65 × 6 = 1230 and 35 = 1830.1 mark
  2. Numerators 13, 14, 15, 16, 17 lie between: 1330, 1430 = 715, 1530 = 12, 1630 = 815, 1730.1 mark
13/30, 7/15, 1/2, 8/15 and 17/30 (other correct sets exist, e.g. 0.41, 0.45, 0.5, 0.55, 0.59).

Check: Decimals: 0.4 < 0.433 < 0.467 < 0.5 < 0.533 < 0.567 < 0.6 ✓.

Answer to write in the exam

25 = 1230, 35 = 1830

∴ Five rational numbers: 1330, 1430, 1530, 1630, 1730 (= 1330, 715, 12, 815, 1730)

Common mistakes that cost marks

  • Multiplying only the numerators by 6.
  • Giving 2.55 as an answer. Its value (12) is between, but in the form pq both p and q must be integers: write 510 or 12.
  • Multiplying by 5 instead of 6, which gives only 4 numbers between (1125 to 1425).

How this can come in the exam

MCQ (1 mark)

A rational number between 37 and 47 is

  1. 714
  2. 314
  3. 914
  4. 414
Show answer

(A) 714
614 < 714 < 814.

Short answer (2 marks)

Find three rational numbers between 14 and 12.

Show answer14 = 416 and 12 = 816 (1 mark). 516, 616 = 38, 716 (1 mark).

Try one yourself

Find 3 rational numbers between 49 and 59.

Show answer

×4: 1636 and 2036 → 1736, 1836 = 12, 1936.

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