Fill in three of the four values in a/b = c/d, leave the unknown blank, and the solver cross-multiplies to find it.
For a/b = c/d, the products a×d and b×c are equal, so the missing term is found by rearranging that equation.
Any one of the four — a, b, c or d. The solver finds whichever value is missing.
Scaling recipes, converting units, map distances, and any situation where two ratios must stay equal.