OK mad gambler here's how you work that out. Say you've got to back three dogs, to profit $100, and they are showing the following odds

A - $3.50
B - $5.00
C - $10.00

Work out each of their percentages, which is done by dividing their price into 100, then turn it into a decimal. So, it is -

A - 100/3.5 = 28 (rounding these) = 0.28 as a decimal
b - 100/5 = 20 = 0.2 as a decimal
c - 100/10 = 0.1 as a decimal.

Total the decimals. In this case, it equals 0.28+0.2+0.1 = 0.58.

Turn that figure back into odds, to do this simply divide the decimal into 1. So, it's 1/0.58 = $1.72.

Therefore, the sum chance of all three of your runners combined is $1.72 that one of them will win.

So, you need to know how much you'd have on a $1.72 chance to win $100. To work that out, divide the price - 1 into 100. In this case: 100/(1.72-1) or 100/0.72, which equals $139. So, you've got to have $139 on to win $100. We can see this is right because if you had $139 on something at $1.72, your collect would be $239, for a profit of $100.

So, your outlay for the race will be $139 to profit $100. Last thing to work out is how much on each dog.

Given that the sum total of their decimal chances is 0.58, then you know that the $139 will be divided across that 0.58 chance. Simple way to work out how much on each, therefore, is divide each dogs decimal chance into the sum of all the decimal chances, and multiply this by the total outlay.

Simply put -

A - dec chance = 0.28, bet size = 0.28/0.58 * $139, or 0.48*$139, or $67.
B - dec chance = 0.2, bet size = 0.2/0.58 * $139, or 0.34*$139, or $48
C - dec chance = 0.1, bet size = 0.1/0.58 * $139, or 0.17*$139, or $24

So, A has a bet of $67, B has $48, C has $24, for a total of $139.

To check the maths, if any of them should win, you'd collect:

A: $67*3.5 = $235, minus the outlay = profit $96 (below $100 because I rounded the decimal at the start)
B: $48 at $5.00 = $240, for a profit of $101 (again, rounding of A to blame for the $1 discrepancy)
C: $24 at $10 = $240, for a profit of $101.

If you'd done the maths to proper decimals etc you'd end up with figures pretty much right on $100, but I couldn't be ************ed typing those decimals.

So, that's how you do it.

Now - the reason this system WON'T WORK, is because sooner or later, the more dogs you have to back, eventually the sum total of your decimals will be greater than 1.0. When that is the case, it is IMPOSSIBLE to back all of them and win. That's just the simple true maths of percentages and odds.

If you don't believe me, go to a TAB website, choose a dog race. Work out how much you'd need to have on #1 to win $100. Then, work out how much on #1 and #2 combined, using the above method. Then, use #1,2,3 combined. Continue in this manner, and you'll see that when the percentages get above 100% (decimal above 1.0), you're SCREWED. LOST your BANK, and possibly your HOUSE.

This system is DANGEROUS, and should not be used, not in ANY form.