It doesn't matter which share are sold.
You buy 100: You have 100 shares.
The next day you buy another 50 shares: You have 150 shares
The third day you sell 40 shares: You have 110 shares
and so on
It's like john is going to the market and buy 3 apples, he goes to another stall and buy 5 more apples, he give 2 apple to his girlfriend: How many apples remain in his basket?
