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In football betting, decimal odds are most commonly used, especially in European countries, Asia and Canada. These odds indicate how much money you will receive for each unit you bet, including your stake. For example, odds of 2.50 mean that if you bet 100 units, you will receive 250 units back (including your original stake). As for fractional odds, these are mainly used in the UK, where odds like 5/1 mean that you will receive 5 winning units for every 1 unit you bet, plus your original stake.
