It is important to note that when stock splits, your total number of stocks increases but your cost remains the same since you did not have to pay anything extra when stock split happens. In your case, you will need to adjust cost basis per lot. In your case, that means, two lots (One for 10 and one for 20 stocks). You use the price as per stock certificate and adjust the basis ($4 and $4.50 as you calculated). I hope this is helpful.
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TurboTax Expert
