When using Commsec, what happens when i purchase the same stock...

When using Commsec, what happens when i purchase the same stock at different prices?

So lets say i purchase 100 shares of XYZ @ 0.20, then next week i purchase 100 shares of XYZ @ 0.50....how does that show up for me on commsec? is it just averaged?

If i decided to sell some later on, lets say 120 shares, does that mean 100 from the first parcel i purchased @ 0.20 are sold, then the remaining 20 are then sold from my second parcel i purchased @ 0.50?

Sorry if this is a stupid question, but its one of the things i never really understood.

Thanks in advance for any help.