Just confirming something.

If a horse at odds of 20.00 has around 5% chance of winning a race,
if I was to back three 20.00 horses in the one race, would my chances of
one of those winning rise to 15%?

Correct.

You can add such chances together.

Whereas adding odds (the inverse of chance) is nonsensical. Yet too many people like making that mistake.

A horse at $20 though might have a 1% chance or 60% of winning, not specifically 5%. You can't work out genuine chance by price or the shortest price horse should win most races which they don't. They mostly lose. Genuine % chance of any runner regardless of odds is speculation only.

Thanks for the input guys.

Crash, whilst what you say is correct, don't you think it's "strange" that overall the difference between the price and the return is about the bookies take???? i.e just under 50% 0f even money chances win (regardless) just under 33% of 2-1 chances win (regardless), as you go up in odds the gap widens, overall the diff is around 18-20% (surprise surprise), were talking pure mathematics here. Take Favs only and the loss is around 8-10% (regardless) it can be narrowed as we all know by price shopping, but the those statisical facts are interesting in themselves??? .........sort of "sobering in a way" as it suggests that by the time you have backed say 1000 x 3-1 chances, (regardless of how they arrived at) you will lose about 12-15%, same thing with 100 x 4-1 chances, 5-1 etc etc.