Debug,

The commission (5% initially but you can qualify for lower rates based on your weekly turnover) is charged on your NET PROFIT for each race only. So if you won $40 on one runner but lost $10 on another IN THE SAME RACE (ie. you backed 2 at $10/each) then you pay 5% of your $30 net profit. On that race last night where I bet on a runner and then layed it off I only paid the commission on the profit I made (so for every $10 I originally bet I made a $1 profit so ended up with $0.95 after comission).

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"Computers can do that????" - Homer Simpson