Cross Multiplication Calculator

Cross multiplication is the everyday tool for solving a proportion, the kind of problem where two ratios are set equal and you need to find a missing value, and this calculator does it for any of the four numbers. A proportion looks like a over b equals c over d, and it captures a huge range of real situations: scaling a recipe up or down, converting between units, reading a distance off a map's scale, mixing paint or chemicals in the right ratio, working out the height of something from its shadow, or any question of the shape this is to that as something is to what. To use the calculator, fill in the three values you know, choose which of the four you want to solve for, and it returns the missing value along with the completed proportion so you can see the full relationship. The method behind it is simple and reliable. Because the two fractions are equal, multiplying each top by the opposite bottom keeps the balance, giving a times d equals b times c, the cross products. From there a single division isolates whichever value is unknown, so if you are solving for d it is b times c divided by a. That one step replaces the guesswork people often use with ratios and gives an exact answer every time. It is a core skill from intermediate school through to senior maths and into trades and daily life, and it is exactly the sort of calculation where setting it up the wrong way around leads to errors. By handling the setup and the arithmetic, the calculator lets you check your method, learn the pattern, and solve ratio problems with confidence. The formula and a worked example are explained clearly below.

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