Between the trolley driver and his first 10 customers, there are 1+10 = <<1+10=11>>11 people are currently on the trolley
On his first stop, 10 people go on and twice as many people got on at the second stop so 10*2 = <<10*2=20>>20 people got on at the second stop.
So at the second stop he had 11 people on board, 3 got off and 20 got on so he now has 11-3+20 = <<11-3+20=28>>28 people
At his third stop 18 people got off and 2 got on so in total there are 28-18 +2 = <<28-18+2=12>>12 people on the trolley
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