If DeliveryGoal is a string, then just declare it as a string literal: "88%"
If it is a double, then parse the double and add the symbol:
string percent = (val * 100).ToString() + "%";
Alternatively, you can have the ToString method format the double for you:
string percent = val.ToString("P");
See the MSDN page on double.ToString for more formatting options.
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solved Using the % Symbol In c# [closed]