OS X Daily notes Siri herself isn't designed to figure out tipping amounts right off the bat. You'll need to ask her in a specific way, posing the query as a math equation:
What is (tip percentage) of (total bill)? What is 20% of $34.52? You can also give Siri the literal math and get the same response, providing an equation like "15% times 60″. Either way, Siri will report back with the proper math and you'll find the tip under "Result".
Of course, where Siri falls short compared to a dedicated tipping app (I personally like Tip'n'Go Tip Calculator for iOS and Tip Me for Android) is the ability to preview differing tip rates all at once or the option to calculate splitting the bill amongst friends. But in a pinch while dining alone or handling the bill all yourself, Siri's math skills can come in handy!