A tip : place 5 units on a horse ?
what does it mean?

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does it mean if you bet 50 per horse, bet five times as much which is 250 ???

Ah yes, you've answered your own question there. A unit is just a certain amount of money that would be a standard bet size for you. As in your example, if you would normally bet $50 on a horse, having a 5 unit bet would be $250.

I could have won if I bet 5 units on a selection, that is the only winnning horse, the other 3 lost, if I had known what it meant, I could have won so much money

too late, why I am having so bad luck

last time, I didn't bet on a horse, cause the odds is too high, and that one won, big time, over 10 times

so I always miss out on the big wins .....

Yes, if you're going to follow other people's tips and not make your own selections, then I guess you really have to follow them to the letter.

I really think you should possibly stick around here for a while and try to learn a bit more about racing/punting, before you start betting too much money.

what does it mean by 5 units each way, does it mean ? if 5 dollars is one unit
does it mean 25 ON WIN, 25 ON PLACE TOTALL 50 DOLLARS or just 25 total then half 12.5 each way?

$25 Win, $25 Place