How does LP gain work?

Got promoted to Silver lll and won a game and gained 19 LP, won another game but only earned 9 LP this time. Took a long break from the game so I have no clue how LP gain works now, if it was even changed at all. Thanks Edit: Forgot to mention that at the time of my first win I had 0LP, so I went from 0 to 19LP. Also forgot to mention I won the games back to back.
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