Proportion Calculator

1. Enter Three Values: Type numbers into any three of the four boxes ($A$, $B$, $C$, or $D$).
2. Leave One Box Blank: The empty box is the variable to be solved.
3. Click Solve: View the exact solution via cross-multiplication.

A proportion refers to an equation showing that two ratios are equivalent. Proportions solving has a lot of applications in chemistry stoichiometry, map cartography, scaling of dosages, and retail unit pricing.

Cross-multiplication principle: If A / B = C / D, then A × D = B × C.

Example: A car uses 5 gallons of fuel to travel 150 miles. How much fuel is needed for 360 miles?

1. Set up proportion: 5 / 150 = x / 360.
2. Cross-multiply: 150x = 5 × 360 = 1800.
3. Solve for x: x = 1800 / 150 = 12 gallons.

Direct vs. Inverse Proportions

• Direct Proportion: As one variable increases, the other increases at a constant rate ($y = kx$). Example: Driving time vs distance.
• Inverse Proportion: As one variable increases, the other decreases ($y = k/x$). Example: Number of workers vs time required to paint a wall.