The question you opened
Counting down the stack
You bought in for 200 dollars and finish with seven stacks of 20 chips worth 1.50 each.
How much did you win, in dollars?
$10
Seven stacks of 20 is 140 chips. At 1.50 each that is 210 dollars, so the night finished 10 above the buy in.
Three steps, all small. Seven stacks of 20 chips is 140 chips. At 1.50 each, 140 chips are worth 140 plus half of 140, so 140 plus 70, which is 210. Subtract the 200 dollar buy in and the profit is 10 dollars. Multiplying by 1.50 is worth having as a reflex, because it appears wherever something is priced at one and a half units: overtime hours, half portions, markups, and chip denominations.
The move is add half. To multiply by 2.5, double and add half of the original. To multiply by 0.75, take three quarters as half plus a quarter. Counting stacks rather than chips is the other habit. Casinos build stacks of 20 for exactly this reason, so a table can be read at a glance without touching anything. Seven stacks is one number, and 140 chips is a multiplication rather than a count.
The order of operations matters more than it looks. Subtracting the buy in from the chip count instead of the dollar value gives a nonsense answer of 60, because it subtracts dollars from chips. Any time two different units are in play, convert first and combine second. It is the same discipline that stops people adding a price in cents to a price in dollars.
A note on the scenario. A ten dollar profit on a two hundred dollar buy in is a perfectly ordinary result, and it is worth saying because the interesting sessions are the ones people talk about. Most nights end within a stack or two of where they started, which is precisely why the arithmetic has to be done rather than felt.
Technique: Count chips, then price them