how is the percentage calculated?

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LionMind22

At the below picture from the chessbase app in the highlighted move 5...a6 you can see on the right the percentages of:

white win: 43% 

draw: 29% 

black win: 29% 

Then in the third column there is a percentage which says 57%. What does that mean and most importantly how is it calculated out of the 3 figures above??

I don't seem to get it. 

 

uri65

white win + draw/2