While I was eating lunch today I thought of something else I can add to this thread: How to quickly figure a tip. The general consensus is that a standard tip is 15%, and that's what I'm going to teach you to do here.

First thing you need to learn is how to find 10% of a number. It's easy: just move the decimal place one spot to the left. For example, 10% of $42.00 is $4.20, and the decimal place is just moved one spot to the left.

The second thing you need to learn is how to find 5% of a number. It's easy: just find 10% of the number as per above and take half of it. For example, 10% of $42.00 is $4.20, half of that is $2.10 which is 5% of $42.00.

So, find 10% and 5%, and add that together to get your 15% tip.

Example: Your bill is $23.57. Round that to $24, and find that 10% is $2.40. Half of that is $1.20. Now $2.40 + $1.20 = $3.60, which is close enough to 15% for our purposes. Note that 15% of $23.57 is actually $3.54 but nobody is going to leave some weird amount of change for a tip usually.

Example: Your bill is $140. Find 10%, which is $14, and half of that is $7. Now $14 + $7 = $21 which is 15%.