Brokerage firms may have a default. Common ones are FIFO and "lowest gain."
But the brokerage won't know about your other earnings for the year. Thus you might want to take a "highest gain" if in that particular year your other income will be low and you expect to pay 0% on any capital gains.
Choosing it yourself - assuming you know why to choose - is always best.
To the OP's question:
For tax purposes, your profit is 20*130 - A*100 - B*110 - (20-A-B)*105
If you pick lots, you can have any of
0 <= A <=20
0 <= B <=20
0 <= A+B <=20
So you have a large number of possible answers. Realistically, you probably want either A=20 or B=20, depending on whether you want to maximize or minimize your gain on the transaction.