Odds can be displayed in two ways: Fractions or Decimals You will probably be most familar with odds as fractions (e.g. 3/1). However, you can choose to display them as decimals (e.g. 4.00) if you wish. How to Change Odds Format Below is a table high
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That explains the concept in a bit more detail.
In simple terms, it's telling you how much money you'll get back if you bet on that result and it wins.
So for 8/15 (0.533...) odds for City to win, if you bet £10, you'll get £15.33 back (including your £10, so £5.33 profit) if you win. (£10 divided by 15, multiplied by 8)
Whereas at 9/2 (4.5) for Real to win, you'll get back £55 from a £10 bet (£45 profit). (divided by 2, multiplied by 9)
8/15 being such a small fraction means the bookmakers think this is far and away the most likely result.
Check your potential winnings on accumulators, Lucky 15s and all your favourite bets with Paddy Power online bet calculator.
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PaddyPower have a calculator that'll help it make sense if you set it to a Single bet