Here's a trick for computing a 15% tip in your head. Start by calculating 10% of the bill and then add half. For example, if the bill is $24 dollars, a 15% tip would be $2.40 (10%) plus $1.20 (half of 15%), for a total of $3.60.
My tip calculation trick is to just double the first number. This seems to work out in most cases. For the example of $24.00, I would tip $4.00 which would be 16.66%. At the high end, a 29.00 bill would be 13.79%, still pretty close.