Compare contents of two dropdown selections

We have two dropdown columns and we’d like to come up with a percentage based on how alike they are. Each dropdown only has 4 selections each. The options are the same for each.

If dropdown 1 selections are A, B, C, D and dropdown 2 selections are A, B, C, X, the percentage would be 25%. If there are no similar selections, it’ll be 0%, if the selections are the same, it’ll be 100%.

Can anyone help?