Table with Percentage per item

First calculate the number of "1"

Number of 1=COUNTROWS(FILTER(Table;Table[Exist]=1))

if you put in a visual with the product as rows you should have

A 3
B 2
C 4

Now the trick is to calculate how many rows you have, in total and that can easily done with a measure
Number of Rows = COUNTROWS(Product)

Again if you put both measures in a table

A 2 5
B 3 5
C 4 5

Now just make a third measure that does the percentage
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Based on your question, I suggest you to take at least a basic course on how DAX works