This is a nice solution from Andrei of School 205 Bucharest. Thank you Andrei.

Looking at the first race.

If the odds are 2 to 1 against the rhinoceros, then the probability that the rhinoceros will win is 1/(1 + 2) = 1/3. For the hippopotamus, the probability that he will win is 2/(2 + 3) = 2/5.

The total probability (that there is a winner) is 1. So, the probability that the elephant will win is:

1 -1/3 -2/5 = 4/15

So, the odds for him are 11 to 4 against the elephant.