Proportion Solver

Given a/b = c/x (or any of three knowns), solve for the fourth.

Inputs

Result

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General calculation reads

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How Not to Be Wrong
Jordan Ellenberg - math thinking
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The Joy of X
Steven Strogatz - accessible math
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Scientific calculator (Casio fx-991EX)
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How to use this calculator

  • Enter a, b, and c.
  • The calculator solves for x in a/b = c/x.

About this calculator

Two ratios are proportional if a/b = c/d. If three of the four values are known, cross-multiplication solves for the fourth: x = (b · c) / a. This is the foundation of "is the same as" reasoning — recipes scaled by ratio, map distances, mixing dilutions.

Frequently asked

How do I solve a/b = c/d for d instead?+
Same logic, swap labels. d = (b · c) / a, just like solving for x.
What if a = 0?+
You can't solve — the proportion is undefined. The calculator shows "—".
Real-world example?+
"If 2 lbs of beef feeds 3 people, how many pounds for 12?" Set up 2/3 = x/12. Solve: x = (2·12)/3 = 8 lbs.
Does this work with units?+
Yes — as long as the units on the left and right sides match. 2 km / 3 hr = x km / 12 hr.
How is this different from ratio-calculator?+
ratio-calculator simplifies a:b. proportion-solver finds the missing fourth term in a:b :: c:x.

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