You can do it, if you know the right formula. For **A/B = C/x**, you would do **(B*C)/A** to get x.

In your provided proportion x would be **(2*6)/3 = 12/3 = 4**.

This is called the rule of three, it appears that it is not taught in most countries but it is taught here in Vietnam. It's very useful when solving a proportion with just a single unknown, and it's faster than standard cross-multiplication.

I posted the method to solve for "x" in my original post. But that required me to write down the ratio on paper to get x=(2*6)/3 and then for a more complicated values, use a simple calculator to find the value of x. ANY calculator can solve for x.

Since I find reason to solve proportions (ratios) fairly regularly, I wanted a calculator that would allow inputting the ratios directly without using paper to visualize the actions required. Unfortunately, it appears that the FX-300ES Plus cannot do this simple calculation directly.

Wonder if the FX-991ES Plus 2nd edition can do it directly?

**Edited by calc-lover, 19 June 2024 - 06:53 PM.**