Here are two ways you can express this:
"The doctor was offering a 20% chance of being able to walk again after the operation."
"The doctor was offering the odds of one in five of being able to walk again after the operation."
There are probably other ways you can say this too :)

I believe the expression you want is "one in five", meaning one successful operation per five operations. I think "one to five" could also work, meaning one successful operation for every five unsuccessful ones.

Does this make sense? (I hope it helped!)

Cassie

Difficult @0@

>May I understand that "five to one" means one successful operation per 5 operations.
This is actually five successful operations per six operations(five successful, one failed).

It is "Against the Odds".

Explanation below was taken from this source: http://en.wikipedia.org/wiki/Odds

////...the first figure represents the number of ways of failing to achieve the outcome and the second figure is the number of ways of achieving a favorable outcome: thus these are "odds against". In other words, an event with m TO n "odds against". ////

So, saying 1 to 5 means that there is a very good chance, using this format.

Using 1 in 5 is saying the same meaning as 5 to 1. But 5 in 1 does not mean the same as 5 TO 1.
For using IN, the smallest figure comes first; 9 in 10, 10 in 25, but not 25 in 10 which you can not take 25 from number of 10.
This is similar to a standard math expression which proper symbol should be used.

Odds of five to one (5:1) means that out of six operations, five are successful and one is not. So five to one is actually 5/6, or 83%, not 20%.

Odds of one to five, on the other hand, means that out of 6 operations only one is successful, which is a 17% success rate.

"Odds of x to y" expresses the ratio of x:y, which is different from x/y.