A Brief explanation of odds
Odds are what bookies use to represent the implied probability of an event occurring, they also determine from a punters point of view how much money you would receive back if your selection was to win.
Example,
£10 on 2/1 returns £30, £20 profit and your £10 stake back
£10 on 3.00 returns £30, £20 profit and your £10 stake back
These odds have an implied probability of 33.33% chance
This bet would give the punter a net return of 200%
Odds are displayed in two different formats, the most common one being fractional 2/1, 7/2, eves and the other being decimal 3.00, 4.50, 2.00 respectively. I find decimal the easiest to use as its easier to quickly work out your winnings without going to the bet slip and clearly outlines the implied probability, the lower the number the higher the implied probability.
Below is a small chart to show an example of the odds in both formats with the implied probability.
When selecting odds for challenges or accas it far easier, in my opinion, to use decimal odds to gauge what your betting on and working out the implied probability.