Proportions, percents, and solving for x
Proportions, percents, percent change and one-step equations in one place — each answer shows the steps.
Result
x = 12
3/4 = 9/x → x = (4 × 9) ÷ 3 = 12
How is this calculated?
Using the tool
The pre-algebra workhorses in one place: solve a proportion, take a percent of a number, find a percent change, or solve a one-step equation. Each answer shows the steps, not just the result.
The math
A proportion cross-multiplies: a/b = c/x means a·x = b·c, so x = b·c ÷ a. A percent of a number is that percent ÷ 100, times the number. A percent change is the difference over the original, times 100. And a one-step equation ax + b = c undoes the + b and the × a to isolate x = (c − b) ÷ a.