How do I calculate my average price? It seems like the stock brokerage is showing a different value.

There are different ways to calculate investment cost and P&L of each sales.

What you described is weighted average method, which gives you the average cost of your current holdings taking into account the profit/loss of your sales.

Another method is FIFO method, which is what @Dex described, where the calculation is based on selling what you bought first.

Hmm, what brokerage are you using? Dont forget to factor in the comm+platform fees if theres any, as that will also affect your average cost. Also, it is likely that you will sell the first 5 shares that you bought first, ie, the ones that you bought at 10@$200, which means you're left with 5@$200 + 10@$100. I could be wrong but that's what i think, cheers.