You don't have enough information to work out the solution. But this is not how to use probability to answer questions in horse racing. You want to ask the following questions (or something similar).

1. What is the percentage of last start winners to all horses ?
This will give you your expected random chance of winning.

2. Do last start winners win more or less then their random chance of winning ?
I already know this is more.

3. What are the designated chances of last start winners by punters ?
Get this by 1/Odds and add them all up.

4. Do punters already factor into the last start winner ?
determined by checking the 1/Odds against actual winners. The answer is yes. (you will find 1/Odds still falls close to the actual number of winners).

From the above you are going to learn 2 things about every form factor you do it for. Question 2 will tell you whether or not the form factor is useful in identifying winners as far as strike rate. Question 4 will tell you whether or not other punters already know this.

The most important thing is Question 4 as this is where the profit comes from but note that you also need a good strike rate. Its good to mix a little that go well in question 2 and question 4.