Your calculation gives you the ratio for every single row. What you are probably looking for is the SUM of all [Ans W/SL] divided by the SUM of all [Offrd] in the partition.
Without having all the details my guess would be that the following calculation will probably resolve your problem.
(SUM([Ans W/SL]) / SUM([Offrd])) * 100
Thank you @Michel Caissie. I have to get Excel out of my head when using Tableau. Thanks for the quick response.