Solving 3/4 = x/12

Pre-Algebra · Axiom Academy

EXAMPLE Solving Proportions with Cross-Multiplication Solve the proportion step by step using cross-multiplication. Two fractions are set equal to each other: . Find the value of x that makes this proportion true. Nice work — you solved a proportion using cross-multiplication. The moves to remember: Cross-multiply: multiply each numerator by the opposite denominator, giving . Simplify the known side: , so the proportion becomes 36 = 4x . Isolate the variable: divide both sides by 4 to get x = 9 . Always check: substituting x = 9 gives , matching the left side. Cross-multiplication turns any proportion into a one-step equation — you will reach for it constantly in algebra, geometry, and any problem involving ratios and rates.

This is the written version of the interactive lesson above. See the full Pre-Algebra course.