Fractions and Odds

To convert odds to fractions: a to b is b/ (a+b)
To convert fractions to odds: a/b is (b-a) to a

Odds are a ratio of non occurrences to occurrences (losses to wins).
Ratios reduce to lower terms [i.e. (9 to 6) = (3 to 2)].

Think of things in terms of fractions or percentages not odds.
3 to 1 shot: 1/ (1+3) = 1/4: 0.25 = 25%
hence 3 to 2 shot = 40%
4 to 1 shot: 1/ (1+4) = 1/5: 0.20 = 20%

Odds need to be converted to fractions to do any arithmetic. Change odds to percentages, add the percentages, and revert that to odds. (i.e. to parlay 3 to shot and 4 to 1 shot):
3 to 2: 2/5
4 to 1: 1/5
1/5 x 2/5 = 2/25, which reverts to 23 to 2 shot.

Percentage advantage is not the same as percentage disadvantage. If you know either then you can tell if your expectation is positive or negative. Your average profit per game is your expectations. With 9 to 1 odds, the house has a 1% advantage, for example each $110 bet makes them $1, yet the player disadvantage is to lose $1 for each $11 bet, which is 9.1% when he bets $1 per roll for eleven rolls where he wins once ($9) and they win $10.