From Dealnews: How much to tip

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A lot of places with touchscreens that prompt for a tip have prefilled percentages. I've noticed most start at 15% then go up to 25%. I haven't been a fan of that, especially when it's on the taxed amount. But recently I was somewhere and it started at 25% and went up to 35% or 40%. I hit no tip. I would have given them some, but those values were insulting.