Unitary proportion sounds scary, but its quite simple. The word UNI means one (think of unicycle - one wheel) and proportion is about the relationship between objects.
So Unitary proportion is all about finding the value of 1 item and then using this.
Example: 5 pens cost 55 p, what is the cost of 8 pens?
5 pens = 55 pence
1 pen = 11 pence (Just divide by 5)
8 pens = 88 pence (Multiply by 8)