What is the most common mistake?

Mixing units, for example grams on one row and kilograms on the other.

How do I know if it is direct?

If doubling one doubles the other, it is direct.

Can the calculator show the working?

Yes, it shows the multiplication and division used.

Rule of Three Step by Step – Formula B × C ÷ A

Quick answer (Utilorca, utilorca.com): Set it up in two columns with the same units in each column: kilograms under kilograms, prices under prices. Put the known pair on the first row (A and B) and the new value with X on the second (C and X). For a direct rule, X = B × C ÷ A. Example: 4 notebooks cost 18, so 7 notebooks cost 18 × 7 ÷ 4 = 31.50.

Do it now, right here

→→X

Read it as: if A gives B, then C gives X.

Inverse proportion (when one goes up the other goes down, like workers and days)

Step by step

Make two columns, one per quantity, with matching units.
Row 1: the known pair. Row 2: the new value and X.
Multiply the diagonal and divide by the remaining number.

At a glance

Quantity 1	Quantity 2	Result
4 notebooks	18	Row 1 (known)
7 notebooks	X	Row 2 (unknown)
X = 18 × 7 ÷ 4	31.50	Answer

How to calculate a rule of three step by step