Aaaah ok you want to dutch your top 3 to make $50 profit if any of them win. Right well thats a little more complex and requires several forumals...

It's a multi-step process.

1) Find the probability of each runner. This is just 100/odds. Eg a 2.0 runner = 50% probability.
2) Sum the probablity for each of the runners (it should/must be less than 100).
3) Calculate the "advantage" in your favour(100 - total probabilty from step 2)

Then for each runner you can plug in this formula...

(100/Odds)*(50/advantage)