In an ideal case, supposing you are playing as White and takes "no time" to make move one, 20 minutes on your time should be used up (assuming an arbiter would have started White's clock at the start of the game), leaving you with 1 hour and 40 minutes, while Black's clock starts running after you have pressed the clock (at 7:20 pm). Black arrives at 7:50 pm, 30 minutes after the clock has been pressed so Black's time should be 1 hour 30 minutes.
Supposing you are playing as Black, you would have been waiting for White to make a move, so your clock would not have run (and an arbiter would have started White's clock at the start of the game) and White's clock would have run for 50 minutes, leaving White with 1 hour 10 minutes while Black has the full 2 hours.
So the round started at 7:00 pm. I arrived at 7:20 and my opponent arrived at 7:50. This was a G120 game.
How much time should be on the clock when my opponent shows up?
I thought I should have 5:20 and my opponent 5:50, but he thinks I should have 5:10 and him 5:40.