Rule of Three
Solves direct proportion: finds unknown x from a : b = c : x.
x
20,00
a : b = c : x → x = b×c/a
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Formula
a : b = c : x → x = (b × c) / a
Example: 2 : 5 = 8 : x → x = 20
Methodology
For direct proportion a : b = c : x, so x = (b × c) / a.
Example
5 kg for 250 CZK → 8 kg: x = (250 × 8) / 5 = 400 CZK.