3 HORSES PETER , JAMES , JOHN ARE IN A RACE IF PETER IS TWICE LIKELY TO WIN AS JAMES AND JAMES IS TWICE LIKELY TO WIN AS JOHN
Assuming that one of the 3 must win,
let the prob(john winning) be x
prob(james winning) = 2x
prob(peter winning) = 4x
x + 2x + 4x = 1
7x = 1
x = 1/7
So now you can find the probability of each horse.