Twice the number of participants in 2018 is 150 participants x 2 = <<150*2=300>>300 participants.
The number of participants in 2019 was 300 participants + 20 participants = <<300+20=320>>320 participants.
Half the number of participants in 2019 is 320 participants /2 = <<320/2=160>>160 participants.
The number of participants in 2020 was 160 participants - 40 participants = <<160-40=120>>120 participants.
There were 320 participants - 120 participants = <<320-120=200>>200 more participants in 2019 than in 2020.
#### 200