Solve proportions using cross-multiplication, simplify ratios, and scale quantities. Enter any three values in a/b = c/d to find the missing fourth.
a/b = c/d → a·d = b·c
A proportion is an equation stating that two ratios are equal. The key property is that the cross-products are always equal: if a/b = c/d, then a·d = b·c.
To find the missing value, isolate it using the cross-product equation. For example, if b is unknown: b = a·d/c.
Proportions preserve the relationship between quantities, which is why they are so useful for scaling, comparing rates, and converting units.
rate problems · similar figures · scale models · unit conversion · percent problems
Proportions appear whenever two quantities change together at a constant ratio. If one ratio is known and one value of the second ratio is known, you can always find the fourth value.
One-on-one Algebra tutoring builds the intuition for cross-multiplication, ratios, and scaling — we work through your actual homework so the method sticks.