Model 6 — Finding the Proportional

04 Try it yourself Interactive

Third against mean

Set two numbers for the third proportional and two for the mean proportional. The walkthrough computes each one both ways, checks both chains with cross products, and finishes with the simplified ratio the combination question asks for.

third prop to 12 and 30, against mean prop between 9 and 25

75 and 15 · ratio 5 : 1

Try 18 and 36 for the third proportional — the multiplier jump is a clean ×2 there, which is when that shortcut is worth much more than the formula.

Press Next to work through it one step at a time.

Third proportional–

Mean proportional–

Their ratio–

Chain factor–

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