Percentage calculation

I want to know the formula for calculating the tick mark percentage.

I have a listing file for 10 products and have 20 customers.

Customers        item 1        item 2        item 3        Item 4          percentage

Customer 1         a                a               r                 r                      50%

Customer 2         r                 r               r                 r                        0%

In the above showed is my table "a" is tick mark and "r" is into mark. I want to get the tick mark percentage calculating formula.

Please help me out on this.

Thank you