Solve a proportion a/b = c/d for the missing value. Leave one box empty and the cross-multiplication is done for you (the rule of three).
Fill in three boxes and leave the unknown one empty.
A proportion says two ratios are equal, written a/b = c/d. If you know three of the four values you can always find the fourth by cross-multiplication, which is the classic 'rule of three'.
Type any three of the four values and leave the unknown box empty. The tool detects the blank and solves for it instantly. If you fill all four or leave more than one empty, it tells you.
Scaling recipes up or down, converting map distances, adjusting mixing ratios, working out unit pricing, and any everyday 'if this, then how much of that' question.