TF2 math is actually very simple but can be confusing due to common abbreviations, dependence on third-party entities and price manipulation by those entities. TF2 currency is Keys and Metal, which is often called "Pure" (an evolution of advertising items for purely key and metal offers)

The basic currency of TF2 is metal. There are Scrap, Reclaimed and Refined Metals. Metal can be crafted and smelted into these different amount interchangeably; as such,

• There are three (3) Scrap in one (1) Reclaimed
  
• There are three (3) Reclaimed in one (1) Refined

Thus,

• There are nine (9) Scrap in one (1) Refined

When pricing items in metal, amounts like 1.22, 3.66 and 12.11 are used. This is in order to save space because when you divide one (1) Refined into its (9) Scrap you find that one scrap is worth 0.1 repeating (0.111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111...)


For example:

If you are buying items for two (2) Scrap each and you buy sixteen (16) items:
The correct math would be: 2 Scrap each x 16 items = 32 Scrap. And since there are nine (9) Scrap in one (1) Refined, divide the 32 Scrap by 9 to get 3.55 Refined (which is actually 3.55555555555555555555555555555555555555555555555555555 repeating)
The incorrect math would be: 0.22 each x 16 items = 3.52 Refined
We know this because by choosing to round a Scrap Metal before doing the math, you end up rounding twice and changing the final answer. For example:

• 0.22 each x 16 items = 3.52 Refined
  
• 0.222 each x 16 items = 3.552 Refined
  
• 0.2222 each x 16 items = 3.5552 Refined
  
• 0.22222 each x 16 items = 3.55552 Refined

This proves that rounding before the equation changes the outcome and is not the correct way to do math.

The secondary currency of TF2 is Keys (typically Mann Co. Supply Crate Keys, but most types of Keys are considered fairly interchangeable).

When pricing items in Keys, amounts like 1.36, 3.52 and 12.8 are used. This is in order to save space versus writing out the Key amount plus the Metal amount. Deciphering the numbers into usable currency amounts confuses many players, but the math is quite simple:

• First, determine an exchange rate -or "price"- for Keys in Metal. For this example, 2.33 Ref will be the price of a Key
  
• Next, multiply the price in Keys -for this example, 3.6 Keys- by the price of Keys in Refined Metal:
  3.6 x 2.33 = 8.4 Refined
  NOTE: Because the "price" of Keys is often manipulated by third-party websites, prices in Keys will not always work out perfectly like basic Metal math will. It is up to the trading parties to come to an agreement on the exact exchange.