If you've ever been to our store and talked about sizing or size adaptation with one of us, we've probably told you about the rule of 3! It's almost become a passion for us!
What is the rule of 3?
The rule of three (also called the cross product rule) is a simple method in mathematics that allows you to solve proportion problems. It is useful when you want to find a missing value knowing that two things are proportional to each other.
How It Works
If we have two quantities that are proportional, we can use the rule of three to calculate a missing quantity.
For example, let's say we know that:
- a is to b as c is to x .
In proportion, it is written like this:
find x , we apply the rule of three with this formula:
Let's imagine that we are looking for how many stitches we will need to cast on to make a blanket 60 cm wide, knowing that we have cast on 30 stitches to make our 10 cm wide sample.
- We know that 30 stitches (a) cover 10 cm (b).
- We want to know how many stitches we need to cover 60 cm (c).
We use the rule of three to find x (the number of stitches):
So, we will need to cast on 180 stitches to get 60 cm.
How to Apply the Rule of Three
- Find the proportions : Identify the two values we know (here, 30 stitches and 10 cm) and the value we want to find (the number of stitches for 60 cm).
- Relate them : Write the proportional equation.
- Calculate the unknown : Cross multiply and divide to find the missing value.
It's really simple and effective for finding the right size of patterns according to our measurements, knitting simple projects according to our samples and adapting patterns to the right size (provided you can adapt the decreases/increases, which is not so easy!)
