I can't think of a single restaurant like that. Almost all of them have computer tickets, and if they do not the prices are always listed on the check. All people have to do is take the price of what they ordered... say 18 dollars for an entree and 7 dollars for a Martini (25 dollars) and multiply it by the tax rate and a tip percentage. Tax here is 9%, and most people tip 15-20%. So, just multiply what you ordered by either 30% or 25% (and take a small portion off due to rounding). Very simple and does not require a calculator or much time. If you are tipping 20% (30% total including tax) you would simply take 10% of the 25 dollars (2.50) and multiply it by 3. 7.50+25. So, your argument is that people can't do 3rd grade math?
Even with a tax rate of 7%, you can get plenty close by doing simple rounding. Any adult should be able to do it in their head. Say you had 38 dollars and had tax of 7% and wanted to leave a tip of 15%. Easy as pie. 22%... 20% of 38 would be 7.60. So, just add a little for the sake of rounding. Make it 8.50 and you'd be close to the actual figure. 8.50+38 equals 46.50. How is that even remotely difficult or time-consuming? I don't get it.
Why are you saying you have to make change? Round up to easily calculable amounts like halves or whole dollar figures. People might pay an extra dollar or 50 cents, but that is a lot less than paying 60 bucks when all you had was soup and coffee.