Why the cross-multiply step works

In direct proportion, two ratios stay equal. If rac{a}{b} = rac{d}{c}, then both describe the same scale factor.

Rearranging the equality gives d = rac{a cdot c}{b}, which is the missing value this tool computes.

Use the scale factor card to confirm whether the second pair is larger or smaller than the first.

Fast examples

  • Price per item staying constant as quantity changes.
  • Map distance and real distance under a fixed scale.
  • Recipe ingredients increasing by the same multiplier.

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