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The World of Numbers

Every number so far is one integer over another

7, −3, 2¼, 65% and 0.375 look different. Here 2¼ means two and a quarter. 65% means 65 out of 100. Are they different kinds of number?

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Each of these numbers is a point on one line.

One shape underneath

No. All five can be written as one integer over another.

Write each one as an integer over an integer. An integer is a whole number or its negative: 7, 0, −3. The bottom number is never 0. Dividing by 0 has no answer.

  • 7 is 7/1.
  • −3 is −3/1.
  • 2¼ is 9/4, since 2 × 4 + 1 = 9.
  • 65% is 65/100. Cut down (divide top and bottom by 5), it is 13/20.
  • 0.375 is 375/1000. Cut down (divide top and bottom by 125), it is 3/8.

A number you can write this way is a rational number. It is a ratio, an integer over a non-zero integer. Write any minus sign on the top.

Five numbers, each rewritten as one integer over another
As it cameAs a pair of integersCut down
77/17/1
−3−3/1−3/1
2¼9/49/4
65%65/10013/20
0.375375/10003/8

Lowest terms means the top and bottom share no factor above 1. A factor divides a number with nothing left over. Divide both by their highest common factor to reach it.

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This track

  1. 0 Start here
  2. 1 Every number so far is one integer over another
  3. 2 There is always another one in the gap
  4. 3 Which fractions stop, and what decides it
  5. 4 A block that repeats is the number written out
  6. 5 Turning a repeating decimal back into a fraction
  7. 6 Checkpoint: fractions and the decimals they make
  8. 7 A length you can draw and cannot write
  9. 8 The argument that no fraction can do it
  10. 9 A spiral built one right triangle at a time
  11. 10 Squeezing it between two fractions
  12. 11 Checkpoint: the numbers that are not fractions
  13. 12 One number can have two decimal names
  14. 13 A block that turns round: 142857
  15. 14 The decimal tells you if a number is rational
  16. 15 The big picture
  17. 16 Track quiz

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