Mathematics · pattern A, Numeric fields
Enter three values of a : b = c : d to find the fourth, with the inverse-proportion answer alongside for problems where more means less.
if a ÷ b = c ÷ d then ad = bc
Cross-multiplication turns a proportion into a linear equation. The inverse case, where the product rather than the ratio is constant, uses ab = cd instead.
28.000000
If four workers take six days, eight workers take three — more means less, so the product is constant rather than the ratio. Deciding which model applies is the actual content of most proportion questions; the arithmetic afterwards is trivial.
If the number is not the part you are stuck on, that is what the service is for — a specialist who explains the working, not just the answer.
Arithmetic runs in double-precision floating point, so results beyond about fifteen significant figures are not exact. Where a question wants an exact fraction or surd, keep the exact form rather than a decimal.