Sorry about that.

To understand the percentages go to the odds calculator available on this site or work them out yourself.

Example : Horse A 4/1 or 5.00 = 100/5 =20%

This is all you need to do. Bookies will always set their market to well over 100%.

When you mentioned dutch betting I assumed you would be betting to the available prices. ie 20 @ 5.00 returns 100 etc. This way you win the same amount regardless of which horse wins.

When I say you would need your average outlay to be less than the equivalent % in my previous post You would work out your outlay as follows:
A 20@5.00
B 10@10.00
C 25@4.00
D 14@7.00

This makes your total outlay 69% or 69 units so if one of these horses wins you get back 100. So you win 31. Too easy you say. Well it might seem so but if over 100 races this was your average outlay (69) but you only collected 100, 63 times, you have outlay 6900 to return 6300.

I hope this makes it a bit clearer.