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Number and algebra basics・Form 3 maths

Ratios a : b : c

A standard method from Number and algebra basics: the steps and worked examples.

Steps

  1. Make the shared term the same. For a : b = 2 : 3 and b : c = 4 : 5, make b equal to 12.
  2. a : b = 8 : 12 and b : c = 12 : 15, so a : b : c = 8 : 12 : 15.
  3. Check by simplifying back: 8 : 12 = 2 : 3 and 12 : 15 = 4 : 5.

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Worked examples

Example 1: Ratios: given a : b = 2 : 3 and b : c = 4 : 5, find a : b : c.
b is 3 parts and 4 parts; L.C.M. is 12
a : b = 2 : 3  = 8 : 12
b : c = 4 : 5  = 12 : 15
a : b : c = 8 : 12 : 15

For the whole topic, read Number and algebra basics: key points, common mistakes and how the DSE tests it.

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